Ecology of Desert Systems

Foreword That the desert here is mildly austere is certainly true, and yet neither the plants nor the animals live und...

Author: Walter G. Whitford

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Foreword

That the desert here is mildly austere is certainly true, and yet neither the plants nor the animals live under what is, for them, painfully difficult conditions. The vegetation flourishes in its own way. For the desert birds and the desert animals this is not an unfavorable environment. They have had to make their adaptations to the heat and dryness of their land, just as the animals of other climates made other adaptations to theirs. For the jack rabbit and the ground squirrel, as well as for the dove and cactus wren, this is obviously a paradise and there is no paradox in the smile which the face of the desert wears. Only to those who come from somewhere else is there anything abnormal about the conditions which prevail. The Desert Year, Joseph Wood Krutch

Deserts are often perceived as extreme, barren landscapes that are seemingly inhospitable and capable of sustaining only the most rudimentary plant and animal life. Hence, deserts have long captivated the imagination of both layperson and scientist alike. Such was the case for author, naturaUst, and philosopher Joseph Wood Krutch who, in the early 1930s, made his first excursion to the American Southwest, a trip that would forever change his life. Krutch, a New Englander and self-proclaimed 'city man,' acknowledged that this journey was taken 'without much enthusiasm,' so he was undoubtedly contemplating considerable misgivings as his train pulled into the dusty depot at Lamy, New Mexico. However, during the next several weeks, as he crisscrossed New Mexico, Arizona, and parts of California and southern Utah, he became enamored of the enormous diversity of vistas, landforms, plants, animals and human cultures of these drylands. Krutch amassed a volume of enduring impressions of these arid landscapes, and yearned for more: '. . . I was still only a traveler or even only the traveler's vulgar brother, the tourist. No matter how often I looked at something I did no more than look. It was only a view or a sight. It threatened to become familiar without being really known and I realized that what I wanted was not to look at but to live with this thing whose fascination I did not understand.' Years later he returned, to spend a single year in the Sonoran Desert, fulfilling a desire to gain a degree of intimacy with this alien land. His delightful book. The Desert Year, chronicles this quest.

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Foreword

Several decades later, another New Englander and 'city man,' made his first trek to the American Southwest. Walter Whitford, trained as an animal physiologist in Rhode Island, was about to start his first job as assistant professor of biology at New Mexico State University. When he arrived in Las Cruces, Whitford was most likely entertaining his own set of misgivings. While embracing the opportunity to study a system so unlike the eastern forest and coastal ecosystems to which he was accustomed, this stark and unfamiliar land undoubtedly made him feel uncertain, much like 'the traveler's vulgar brother, the tourist.' However, like Krutch, the desert would forever change his life. As a naturalist, Whitford immediately took to the field (often at high speeds in old trucks), seeking to observe and understand the geology, flora, and fauna; as a scientist, he conducted experiments and measured everything he thought made deserts disdnctive. Over the next forty years, Whitford would amass an impressive volume of work. This book. Ecology of Desert Systems, is witness to his duel role of scientist and naturalist, an anthology of a life's work striving to gain an intimacy with desert ecosystems. Initially, Whitford viewed the varied organisms of the northern Chihuahuan Desert through the lens of a classically trained physiologist. However, in the 1960s, ecology was undergoing a revolution of sorts with the emergence of ecosystem ecology as a discipline. The International Biological Program (IBP) was launched in a bold attempt to understand how entire ecosystems function and, by modeling the behavior of these systems, predict how they would respond to environmental stresses. Whitford organized an IBP study to understand water and nutrient dynamics in a small desert watershed north of Las Cruces and, drawing from his intuitive understanding of natural systems, adopted a holistic approach combining empirical observations with theoretical constructs - to forge a systems view of the complex workings of deserts, one of the very first attempts to apply this perspective to aridlands. He followed his curiosity and expanded his own research queries to an extraordinarily broad set of topics, including nutrient cycling, plant productivity, microbial ecology, and rodent population dynamics, all in the context of a systems understanding. This remarkable breadth is at the core of the Ecology of Desert Systems. Considering that nearly 20% of the world's population and one-third of its land surface is located in arid and semiarid regions, linking ecological knowledge to the economic and social consequences of climate change and other anthropogenic disturbances - and to concerns about sustainable land management - is crucial. Land degradation in arid systems, which involves many complex interactions and feedbacks between human social structures, soils, vegetation, and biogeochemical and hydrological processes, is a complex phenomenon that eludes simple characterizations. Nevertheless, it is clear that ecologists will only be able to make substantial contributions to the debate if armed with strong conceptual models of how arid lands work and how people change them. As suggested by the title, the Ecology of Desert Systems is one such conceptual model, written by an expert with a naturalist's vision that also embraces a systems perspective, and provides a

Foreword

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dynamic 'blueprint' for the understanding of the ecology of arid lands. Whitford provides a set of simple but powerful guidelines for managing aridlands: that short-term dynamics are not necessarily indicative of longer-term phenomena; that the behavior of desert ecosystems is wonderfully complex and all too often difficult or impossible to predict; and that humans have major impacts on these 'simple' ecosystems, often in unknown and unpredictable ways. During his year living in the desert, Krutch recognized both the beauty of his surroundings and their subtle complexities: There is all the difference in the world between looking at something and living with it. In nature, one never really sees a thing for the first time until one has seen it for the fiftieth. It never means much until it has become part of some general configuration, until it has become not a 'view' or a 'sight' but an integrated world of which one is a part; until one is what the biologist would call part of a biota.

The Ecology of Desert Systems is an affirmation of the difference between 'looking at something and living with it.' James F. Reynolds Duke University Durham, NC

Preface

No wind. The heat bears down. It has not rained for one year. I love this dry land Am caught even by blowing sand, reaches Of hot winds. I am not the desert But its name is not so far from mine. Extract from 'Desert Cenote' by Keith Wilson in Bosque Redondo: The Encircled Grove, Penny whistle Press. (Cenote is Spanish-Aztec for 'water hole, oasis')

If I had the talent of my friend Keith Wilson, I would have expressed my love and fascination with deserts in poetry. Since I do not possess that talent, I have resorted to narrative of a scientific nature to communicate a picture of the harsh reality of life in the wonderful dry places of the planet. I was encouraged to write this book by several colleagues and students. Students enrolled in my desert ecology classes provided critique's of early drafts. Without their encouragement, I would never have attempted the daunting task that this book represents. This book was written for them and for future generations of students. It is hoped that this book will encourage those students to address many of the fascinating questions that this ecologist could not address in his lifetime. 'Ecology of Desert Systems' is the culmination of nearly 40 years of fascination by deserts and the organisms that reside therein. That fascination has maintained my enthusiasm for research on all aspects of desert ecology to the present day. The completion of the book was accomplished after I was no longer bound by the requirements of gainful employment. The support for continuing research and collaboration with colleagues and students postretirement has been afforded me by the US Department of Agriculture, Agricultural Research Services's Jornada Experimental Range in cooperation with New Mexico State University. I could not have completed this book without that support. Most of my research for the past 38 years has been conducted on the Jornada Experimental Range. The Chihuahuan Desert of southern New Mexico has been my frame of reference in my work in other deserts of the world. The Jornada Experimental Range has provided the baseline for comparisons and contrasts of

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Preface

the Chihuahuan Desert with other arid lands. Although I have attempted to write this book in a global context, my familiarity with the Chihuahuan Desert has undoubtedly dominated my thinking about deserts. Despite this, I hope that the reader will find this a suitable springboard for their own research and thinking about deserts. The ideas and concepts developed in the 'Ecology of Desert Systems' result from collaboration and discussions with many desert scholars. Many of the authors of papers cited in the book are colleagues, postdoctoral associates and students from whom I learned a great deal at conferences, in the laboratory, and in the field. My mentors and students led me to research topics in desert ecology in which I had no formal training. My colleagues in the Biology Department at New Mexico State University, William Dick-Peddie, Ralph J. Raitt, and James R. Zimmerman provided much needed postdoctoral mentoring by generously allowing me to tag along on field trips and patiently identified the plants, animals, and landscapes which were all new to me. Visitors and colleagues who had a large impact on me and served as role models in my career include: John CloudsleyThompson, George Ettershank, Robert Chew, William Nutting, Clifford Crawford, Claude Grenot, John A. Ludwig, Gary L. Cunningham, Mark Westoby, M. Timm Hoffman, Susan Milton, W. Richard Dean, Graham I. H. Kerley, and John A. Wallwork. I owe special thanks to David Tongway of CSIRO Wildlife and Ecology, who spent hours discussing the relationships between patch dynamics and landscape structure and landscape processes in the laboratory and while driving the hundreds of kilometers between the CSIRO laboratory in Deniliquin to Lake Mere. He influenced my thinking about desert ecosystems and landscapes more than any other individual. David made my visits to the Lake Mere research site in northwestern New South Wales intellectually stimulating and enjoyable.

Chapter 1

Conceptual Framework and Paradigms

Nearly one-third of the land area of this planet is classified as arid or semiarid. These lands are areas where rainfall limits productivity and/or is so unpredictable that cropping is not feasible. Of the approximately 37,000,000 km^ classified as arid and semiarid lands, 25,560,000 km^ are used as rangelands: lands used by pastoralists for domestic livestock production (Verstraete and Schwartz, 1991). Despite these Hmitations, humans have inhabited these lands for millennia, using the limited and varied productivity to support pastoralism. In the 20th century, nomadic pastoralism has been replaced by pastoral industries in many areas of the world. Commercial livestock production has had very different impacts on arid lands than nomadic pastoralism. Only the hyper-arid regions (those areas receiving less than 80 mm of rainfall per year (UNESCO, 1977) are excluded from the desert rangelands. Nearly coincident with the development of commercial ranching or pastoral industry, was the realization that these lands were fragile and that when degraded, recovery was slow or did not occur (Dregne, 1986). Along with the recognition that degradation of rangelands was occurring, came the realization that we lacked sufficient knowledge about how these rangelands worked to develop sustainable management strategies. During the last half of the 20th century, a considerable literature has developed on most aspects of the ecology of deserts and of desert organisms. Despite this explosion of information, problems of sustainable use and management of arid lands remain nearly as intractable as they were a half century ago. In economically developed regions of the world, desert lands are viewed by humans not only as places for the production of harvestable animal products but also as regions that support diverse landscapes and floras and faunas. These lands are considered resources that provide opportunities for recreation and escapes from the hectic pace of cities. These changes in attitude toward 'wastelands' have generated conflicts among different interest groups. Therefore the problems of managing and sustaining desert lands have not disappeared nor are they any more tractable now than they were a half-century ago. Buzz words like sustainable ecosystems have entered the lexicons of many people but what they mean and how these words will lead to achieving that desired state still eludes us. A conceptual framework that incorporates the information from population to ecosystem levels of organization at appropriate temporal and spatial scales may provide a blueprint for understanding arid lands.

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Ecology of Desert Systems

1.1 PATTERN AND PROCESS Studies of ecological systems must focus on both pattern and process. Pattern may be structural or temporal. Processes are mechanisms, many of which are similar in ecological systems that are structurally different. Ecological systems are in many ways analogous to an automobile. Understanding how an automobile moves down a highway involves understanding the structural components in relationship to each other and the processes of combustion of fuels and expansion of gasses that move pistons within a cylinder. Other processes involve the transfer of energy to the drive shaft and axles via gears in order to provide sufficient torque to overcome the resistance of the mass of the vehicle. The structures of a diesel automobile are different but the basic processes (expansion of combusted gasses and gearing) are similar. Included in the efficient functioning of an automobile is the temporal pattern of firing sequence to provide a smooth transfer of energy to the drive chain. In ecological systems, temporal patterns of variables such as rainfall and temperature drive the sequencing of processes such as nutrient cycling and primary production. Although the automobile analogy provides some insights into what is needed to understand ecological systems, the analogy is limited. The patterns and processes of an automobile are sufficiently well understood for predictive models (for design and construction) to work. However, for ecological systems, structural complexity, variable time scales, and the variable spatial scales of important structural features contribute to the difficulty of developing predictive understanding of ecological systems. Understanding the ecology of desert systems requires understanding both pattern and process and their relationships. Ecology is generally considered at eight levels of organization: cell, organism, population, community, ecosystem, landscape, biome, biosphere. However, our thinking about ecology and ecological relationships must not be constrained by a conventional hierarchy. It is necessary to keep temporal and spatial scale ordering separate from ecological organization constraints. Scale ordering of time and space addresses the physical side of ecological systems. Both scale-dependent physical fluxes and human intellectual constructs such as community, ecosystem and landscape are necessary to address the full range of phenomena that contribute to the ecology of desert systems. Ecological processes do not function in a way that is limited to physical and chemical mechanisms (Allen and Hoekstra, 1990). For example, the behavior of organisms in a landscape are distinctive and dependent on the genetic characteristics of the species population. A landscape feature such as a river-bed may be a corridor for some species, a barrier for other species or a neutral feature for others. These behaviors cannot be understood as physical and chemical mechanisms but are important for understanding ecological systems (Allen and Hoekstra, 1990). Communities have historically been identified as discernible aggregations of organisms occupying concrete patches on the ground. However, since communities are human intellectual constructs, they are best considered assemblages of


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organisms that are constantly accommodating and changing in character as the result of natural selection of species to the presence or absence of associates. Communities are the integration of the complex behavior of the biota that produces a cohesive whole. That whole exhibits properties of self-regulation and self-assertiveness that can modify the physical environment. Populations consist of only one species and communities involve many species (Allen and Hoekstra, 1990). Populations are tangible and can be identified with particular places on the landscape at an instant in time. Ecosystems involve integration of water and nutrient fluxes over wide ranges of time and spatial scales. Because ecosystems include the influences of abiotic elements and feedbacks between the abiotic and biotic components, defining ecosystems by an area of landscape is of limited value and may be misleading. As a conceptual construct it is important to define carefully the scale at which ecosystem properties and processes are being considered. Landscapes are composed of distinct, bounded units that are differentiated by biotic and abiotic structure and composition (Pickett and Cadenasso, 1995). Landscape can also be considered as an abstraction representing spatial heterogeneity at any scale. In arid lands, landscape units are generally recognized by the life form and species composition of the dominant species (classical concept of biotic community). For example, creosotebush, Larrea tridentata, shrubland may make up one of the distinct units on a watershed. The L. tridentata shrub canopies intercept wind and rainfall thereby changing the spatial distribution of the water and modifying the energy and characteristics of the wind at the soil surface. This dominant species creates patches with modified microclimates. Within a landscape unit we may recognize several mosaics, that is combinations of plants that are distributed throughout the unit. An example of different mosaics in L. tridentata shrublands are: shrubs with large clumps of the grass, Muhlenbergia porteri, around the base of the shrubs, shrubs with a relatively continuous understory of a grass such as Scleropogon brevifolia\ shrubs within a matrix of several species of bunch grasses and shrubs with no grasses in the understory. Each of these mosaics may be present in a single landscape unit recognized as a creosotebush shrubland. Within mosaics there is a finer scale of landscape unit that can be recognized as patches (Table 1.1). Patches may be unvegetated areas, diggings of animals, contrasting surface materials such as lag gravel or aggregations of stones. Many agents of patch formation are episodic especially in arid regions. Patches may form at different times and places on the landscape thereby producing a shifting mosaic (Pickett and Cadenasso, 1995). At the finest scale, a patch is a single plant and the soil around that plant. The living component of desert ecosystems is composed of many species that are 'living on the cusp of disaster'. Many, but not all, of the species living in deserts are operating in conditions that are very close to their tolerance thresholds or very close to the maxima or minima for limiting factors. When extreme conditions occur (long periods with little or no rainfall, periods of time with temperatures well above or below the average temperature maxima or minima)

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Ecology of Desert Systems Table 1.1 Definitions and Spatial Relationships of Landscape Units from the Patch Scale to the Regional Scale

Unit type Patch Mosaic Landscape Region

Definition A patch is a function of the size of a single kind of plant growing on a homogeneous soil A unit containing contiguous patches A unit that contains several mosaics A number of landscapes within a general bioclimatic zone, may be arbitrarily defined by major drainages etc.

Size < 1-10 m^ < 1 ha to 1 km^ > 1 km^ Hundreds of km^

Based on Reynolds et al. (1997).

there is often local extinction of species for which the conditions exceed their tolerance limits. These episodic climatic events contribute a temporal component to structural heterogeneity. The heterogeneity in structure of desert ecosystems and landscapes is also a product of historical forces acting on the geological structures of a region, i.e. pedogenic (soil forming) processes that form soil patches. The soil patches are further modified by the interactions between soil, climate, plants, and animals. The availability of suitable patches may be extremely important for the long-term survival of species during 'crunch' periods of extreme drought or other episodic event that pushes species to their tolerance limits. The structural characteristics of patches determine how the abiotic environment affects that patch and how the abiotic environment is modified by the structural elements of the patch. For example a patch made up of relatively tall shrubs with large dense canopies interacts with the abiotic environment very differently from a patch composed of three to four grass clumps. The redistribution of rainfall via stemflow and subsequent depth of storage in the soil is very different in these patches, as is capacity to retain litter etc. The structural characteristics affect every process and hence must be considered in any analysis of the functional aspects of patch or ecosystem. Although the patch is frequently the spatial scale at which studies of functional relationships are focused, understanding ecosystem processes requires studies at larger scales. It is not possible, for example, to predict the quantity of water entering an ephemeral stream by aggregating the run-off from the patches that make up the watershed. There are processes at the watershed scale that are unique to that assemblage of patches that make up the cover on the watershed. These processes cannot be derived by simply summing inputs and outputs from the component patches (Table 1.1). Before we can understand ecosystem processes at the patch to landscape scale, it is necessary to examine the characteristics of the structural components and how these components serve as determinants of ecosystem and landscape properties.


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The characteristics of the dominant plant species within a patch, mosaic, or landscape unit determines the way in which that entity interfaces with weather and how the environment is modified by that entity. Life form and morphological characteristics must be considered in terms of general life history characteristics in order to understand the contribution of various plant species to ecosystem and landscape properties. The way in which plants interface with wind, rain, and sunlight thereby modifying the quantity and/or intensity reaching the soil surface is very important in establishing the spatial patterns of soil moisture and air temperature. That interface also affects the physical location of materials such as plant litter and seeds. In deserts the life forms that are most important in interfacing with weather are perennial species of bunch grasses, subshrubs, shrubs, and small trees. Perennial herbaceous plants are too sparse in most deserts to serve as determinants of ecosystem or landscape properties. Annual or ephemeral plants are generally too intermittent temporally to play an important role as a determinant of landscape properties. Perennial bunch grasses interface with weather and landscape level processes such as run-off and deposition. Bunch grasses have dense root mats, which are concentrated under the grass canopy. The density of subcanopy roots and their relationships to the vegetative tillers that make up the bunch varies among species. The rate of root growth and root death and physiological characteristics such as quantities and kinds of materials exuded from roots also varies among species. An important contributor to the variation among species is the morphology of the canopy. Canopy morphology variables that affect the quantity of intercepted rainfall that is translocated to the root system include: leaf length, leaf width, and mean slope angle of the leaf (DePloey, 1982; Van Elewijck, 1989). Some bunch grasses have narrow, short leaves, which may be very erect (high slope angles) whereas other bunch grasses may have more prostrate, narrow, short leaves. The proportion of stemflow water that reaches the soil in the immediate neighborhood of the grass roots is immediately available for growth and production. Effective stemflow to dense root mats may stimulate complex carbohydrate exudation from the roots which can increase rates of carbon turnover and nutrient mineralization in the rhizosphere of the plant. Life history characteristics of bunch grasses have an effect on their importance as determinants of landscape properties. For example, long-lived perennial grasses that reproduce primarily by stolons may have very low seed production. Such grasses may exhibit lags in increasing canopy cover and biomass in response to above average rainfall. If drought is sufficiently prolonged or combined with a stress such as grazing, the plant may die and have produced insufficient seed to be replaced by seedlings during the post-drought period. In such a scenario, the loss of bunch grass canopy of a long-lived grass leaves the landscape unit without perennial cover during the first storms of the post-drought period. The loss of this plant cover exposes the soil surface to the full impact of the raindrops, which initiates soil loss. In mosaics of short-lived and long-lived bunch grasses, some

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Species produce copious seeds (e.g. Sporobolus flexuosus in the Chihuahuan Desert). Following drought such a species can dominate a patch in which it was a minor component during the pre-drought period. 1.2 DEFINITION OF ECOSYSTEM In functional terms, an ecosystem is all of the living organisms in an area interacting with each other and with their physical environment. The interactions include basic ecosystem processes such as energy flow and nutrient cycling, and population processes such as competition and predation. Ecosystem processes and properties also vary as a function of a variety of vegetation structural characteristics. Ecosystem processes are also affected by animal activities and their effects on ecosystem properties such as spatial and temporal heterogeneity in the distribution of essential resources. Interactions such as effects of animals on vegetation structure, soil properties and water distribution are considered as essential modifiers of ecosystem processes. We consider ecosystems as systems because we perceive these units as having properties that are more than a sum of the component parts. System attributes that are more than the sum of the parts develop from the interactions among organisms and interactions between organisms and the physical environment. These interactions produce rate-modifying feedbacks that determine the ultimate responses of component populations through time. Just as mechanical systems require an outside source of energy to move the parts and have that system function, ecosystems require a source of energy from outside the system to drive the processes. If we consider an ecosystem as a functional unit, it has no discrete physical boundaries. We typically select study units from within a reasonably homogeneous piece of landscape (a patch) for our investigations of ecosystem properties, structure, and function. Therefore, most of the studies done to date provide little information about exchanges among distinguishable landscape units and the effects of these exchanges on ecosystem properties and processes. The ecosystem concept however needs no defined physical boundary. Conceptually an ecosystem can be as simple as a tussock of grass, the soil in which it is rooted and the atmosphere above that plant (Fig. 1.1). The ecosystem processes: energy flow, material cycling, and the inputs, and outputs to and from that system are conceptually similar when considering a grass clump or a larger area of landscape. Considering the simple grass clump as an ecosystem, it is apparent that it is dynamic, that is, it is changing through time. As the grass grows, matures and senesces in response to climate and soil conditions, animals feed on the foliage, flowers, seeds, and roots. At the same time soil microflora and microfauna change the chemical and physical properties of the soil. There are direct and indirect feedbacks within the system that modify the state of that system through time. The magnitude and direction of such feedbacks are the result of biotic interactions such as herbivory, predation, competition, etc. (Fig.


.-^lfe,.?jA#'^^»fc

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MIA

Figure 1.1 Examples of a simple ecosystems or patches (delineated by broken lines) with surrounding patches. Arrows indicate direction and magnitude of inputs and outputs. Inter-patch exchanges occur as the result of differences in microtopography.

1.1). The ecosystem does not function without inputs and outputs. Inputs include atmospheric gasses, precipitation, air-borne dust, and materials brought in by mobile consumers; outputs included gasses, wind and water transported materials and materials carried by consumers. The focus of most ecosystem research is on ecosystem structure and function. Structure refers to the more static or compositional features of the system and function refers to the dynamic features. Odum (1962) suggests that there are three elements of structure: (1) composition of the biological community; (2) quantity and distribution of the abiotic materials; and (3) range of physical conditions. Odum (1962) Hsts three elements of function: (1) rate of energy flow; (2)

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rate of nutrient cycling; and (3) ecological regulation of organisms by environment and of environment by organisms. Ecosystem ecologists emphasize the 'system' idea focusing on interactions among components and feedbacks that regulate the functions. The ecosystem concept thus is defined as: 'a system is a complex of interacting subsystems, which persists through time due to the interaction of its components. The system possesses a definable organization, temporal continuity and functional properties which can be viewed as distinctive to the system rather than its components' (Reichle et al, 1975). The implications of such a definition are that there are numerous links and feedbacks among the biotic and abiotic elements of the system that are essential for the temporal continuity of that system as a recognizable system. There is a need to study those connections and feedbacks that glue the system together as an integrated whole as well as to examine the connections and feedbacks of the component subsystems. The structure and functions of desert ecosystems (physically patches and mosaics) should provide a general test of the ecosystem hypothesis (sensu Noy Meir, 1979/1980) and provide the functional framework for understanding relationships between landscape units. Ecosystems are perceived as providing goods and services for the human populations. Exploitation of the goods provided by ecosystems can compromise or change the nature of the services provided by ecosystems. Human society in attempting to understand what is involved in sustaining productive wildlands, especially in arid regions, is faced with inadequate knowledge of the quantitative relationships between ecosystem function and the goods and services needed by society (Table 1.2). The conflicts arise between the segments of society that perceive aridlands as sources of goods and those that perceive arid lands as systems supporting a healthy lifestyle. Understanding ecosystem processes is essential for wise management of ecosystems for either goods or services (Table 1.2). Table 1.2 The Contributions of Arid Ecosystems to Human Society Divided into General Categories of Goods, Service, and Processes Goods Foods, Fibers, Fuels, Medicines, Building materials. Industrial products Services Modification of climate (albedo, evapotranspiration etc.), Development of soils. Degrading wastes, Storing wastes, Natural control of pathogens and parasites. Places for aesthetic, cultural, and spiritual renewal (recreation and nonconsumptive uses) Processes Production of organic matter, Decomposition of organic matter. Nutrient cycling. Redistribution and storage of water (infiltration, run-off, run-on, evapotranspiration). Erosion of soil


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It is the goal of ecosystem science to predict the state of an ecosystem at some time in the future based on the knowledge of a small but selected set of parameters or variables at time zero and given a specific set of climatic conditions, abiotic inputs, and perturbations imposed by human activities. We need to understand how critical ecosystem processes vary through time and the mechanisms causing that variation. Such understanding will hopefully allow us to address ecosystem properties such as stability, resistance to change and resilience with change. Stability can be viewed as oscillations within boundaries through time such that the essential biological features of the system remain unchanged over long periods of time (decades to centuries). Resistance is a measure of the ability of an ecosystem to maintain its structural and functional properties when subjected to a disturbance. Resilience is a measure of the capacity of a system to return to its predisturbance state after it has been changed by a disturbance. In desert ecosystems, an understanding of stability, resistance and resilience is critical. The desert ecosystems of the world are undergoing rapid changes that affect ecosystem processes. These changes are collectively referred to as desertification. Desertification results in reduced productivity and reduced habitability for humans. Reversing or at least halting desertification will require a better understanding of the basic properties of desert ecosystems.

1.3 LANDSCAPE UNITS Ecosystems may be studied and modeled without considering fixed geographical boundaries. However, for most ecologists, boundaries between vegetation types or physical boundaries such as land-water interfaces are inherent in the conceptualization of ecosystems. The boundaries of such units may be coincident with boundaries between soil types or may simply reflect changes in cover and composition of the dominant vegetation on the same soil-mapping unit. However, not all boundaries that are structurally important can be identified by such means (Ludwig and Cornelius, 1987). Watersheds and other landscape units may be subdivided into patches or mosaics on the basis of boundaries delineated by suites of structural properties (Table 1.1). Because biotic composition and quantities of abiotic materials are primary determinants of rates of ecosystem processes, these boundaries must be considered if we are to understand processes within ecosystems. Defining the limits of patches is necessary to examine how the transport of materials and movement of biota across such boundaries contributes to the functioning of a landscape or geographic region. Because the dominant (largest biomass, most frequently occurring species) vegetation is relatively homogeneous within such a unit, it is reasonable to expect that ecosystem processes within the unit may reflect characteristics of the dominant plants, biota and soils associated with those plants. An ecosystem by this definition is equivalent to a 'habitat type' but with the caveat that features of the abiotic environment were integral parameters in characterizing the ecosystem.

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However, many of the important features of desert systems involve more than interchanges and feedbacks between the biota and the abiotic environment. The ecology of desert systems must also address population and community processes such as competition and symbiosis. It is important to keep what is allowed by the structures in our observational framework (population processes, community processes) separate from the unbounded world of continuously scaled fluxes (Allen and Hoekstra, 1990).

1.4 QUESTIONS Questions that focus on mechanisms, i.e. how ecosystems function, are central to ecosystem ecology and ecosystem science. A wide variety of questions evolve from and contribute to this kind of general question. We want to understand ecosystem functioning sufficiently well to be able to predict the state of that ecosystem at some time in the future. That goal is one shared by ecologists whose interest is limited to curiosity about the world with no application in mind and by land managers who may have short-term and long-term goals of commodity production, maintenance of biodiversity or maintenance of ecosystems services as their primary goal. One kind of question asks: what determines the long-term 'stability' of ecosystems that make up a landscape? Stability in the ecosystem sense is generally defined as little change in the rate of ecosystem processes or in the biotic and abiotic components with respect to their equilibrium value (Walker and Noy Meir, 1982). That definition presupposes that undisturbed ecosystems can be characterized by 'equilibrium' values for rates and components and that these may vary within some readily measured limits through time. Although that may not really apply to natural systems, it is a useful working conceptualization for ecosystem study. Indeed, ecosystems may vary around a set of values that changes through time but the short-term result is that of an equilibrium situation. Subsets of the stability question involve what relative proportions of individual species of organisms in the assemblages of plants, birds, mammals, arthropods, etc. will occupy a defined area of land in the future given certain climatic patterns. Related to this, how will the biomass, density etc. of these species or assemblages change through time? We would like to know if assemblages of organisms that make up identifiable and repeating patches on a landscape vary within limits over time and represent 'stable' assemblages or if even in the absence of human intervention the ('n') number of assemblages change in composition through time. A critical consideration is if an ecosystem will retain its stability or sustainability in the face of extinctions and establishment of exotics. Another way of stating that question is 'are all species equal in importance in ecosystem processes and as determinants of ecosystem properties?' Perhaps more useful than the concept of stability is the concept of resilience. Resilience is more precisely defined as a measure of the magnitude of population perturbations or degree of disturbance a system will tolerate before changing to


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some qualitatively different dynamic behavior (May, 1976; Walker and Noy Meir, 1982). What are the ecosystem properties that confer resilience to that ecosystem? The notion that complexity of food webs confers greater stability and resilience to ecosystems is not supported by the data for a variety of ecosystem types. May (1976) concluded that mathematically increased complexity makes for dynamic fragility rather than robustness. However, he modified that by stating its dependence upon the general level of random fluctuation in the environment. If trophic complexity confers neither stability nor resilience to an ecosystem, what are the appropriate parameters to examine as contributors to these ecosystem properties? Arid and semiarid ecosystems must be examined in terms of spatial and temporal heterogeneity of key processes and the organisms involved in these key processes. Arid and semiarid landscapes are complex mosaics of interlocking habitats or units that physically, and/or, by activities of the inhabiting organisms modify the patterns of availability of key resources: water and nutrients within and among units. The stability and resilience of the component ecosystems of desert landscapes must be examined in that larger context if we are to minimally achieve the objectives of predicting future states of the systems given certain climatic fluctuations or imposed management and land use practices. Although there are limited data, it is possible to summarize and synthesize these data to develop a useful conceptual model of arid and semiarid ecosystems. 1.5 DEFINING DESERTS Before initiating a discussion of the similarities and differences among deserts and more mesic environments, it is necessary to set the limits of the temperature and moisture regimes to be considered and the general framework within which these environments are to be discussed. Considering the importance of extrapolation of results from studies, experiments and models from one 'desert' area to another, it is obvious that we must have objective criteria for making such comparisons. Because of the over-riding importance of climate as the force shaping the physical environment and biological characteristics of deserts, the delimitation and definition of areas referred to as deserts must be based on climatic criteria. I have chosen to follow the UNESCO (1977) definitions and delimitation of arid and semiarid regions based on indices of bioclimatic aridity. Bioclimatic aridity increases as water gained by rainfall falls far below that potentially lost by evaporation and transpiration. The UNESCO Man and The Biosphere (MAB) group that produced the UNESCO (1977) map of arid and semiarid zones of the world chose the ratio P/ET (in which P is mean annual precipitation and ET is the mean annual evapotranspiration) as the index of aridity applied to delimit zones of varying degrees of aridity. They chose P/ET because: (1) it gives the same value for all climates in which potential water loss is proportionally the same in relation to rainfall; (2) it is biologically accurate in

12


climates with highly concentrated seasons and (3) it was used by the FAO (Food and Agriculture Organization) in its study of desertification risk. UNESCO (1977) proposed four classes of aridity: (1) Hyper-arid zone (P/ET < 0.03), areas with very low irregular aseasonal rainfall; perennial vegetation limited to shrubs in riverbeds; (2) Arid zone (0.03 < P/ET < 0.20), perennial vegetation is woody succulent, thorny or leafless shrubs, annual rainfall between 80 mm to as much as 350 mm; interannual rainfall variability is 50-100%; (3) Semiarid zone (0.20 < P/ET < 0.50), steppes, some savannahs and tropical scrub; mean annual rainfall is between 300 mm and 700-800 mm in summer rainfall regimes and between 200 mm and 500 mm in winter rainfall at Mediterranean and tropical latitudes. Interannual rainfall variability is 25-50%. (4) Subhumid zone (0.50 < P/ET < 0.75), primarily tropical savannahs, and chapparal and steppes on chernozim soils. The UNESCO (1977) group included this zone in their map because of the susceptibility to soil and vegetation degradation during droughts. The deserts in which the processes described in this book are applicable are those defined by Meigs (1953) as arid or semiarid. The areas that are classified as hyper-arid, have little or no perennial vegetation and are seldom used by humans except as a source of mineral resources hence are considered here only for comparative purposes if data are available. The deserts, which are the focus of this book, are characterized by climates where rainfall rarely exceeds potential evaporation and ambient temperatures remain above freezing for more than 6 months a year. In general such areas receive between 50 mm and 400 mm precipitation per year and domestic livestock production is the primary economic use of the land.

1.6 THE PULSE-RESERVE PARADIGM AND THE AUTECOLOGICAL HYPOTHESIS Noy Meir (1979/80) questioned the applicability of the ecosystem model to deserts suggesting that an alternative and simpler hypothesis might be sufficient for understanding deserts. He offered the autecological hypothesis as the alternative hypothesis. That hypothesis he stated as follows: Tn deserts, the dynamics of each population are determined mainly by its (the population's) independent reaction to the environment, in particular to the highly intermittent availability of water.' He developed a case for the autecological hypothesis using examples for plants both annual and perennial that he identified as 'arido-passive'. He suggested that the 'pulse-reserve' model or paradigm adequately describes the life history and population dynamics of such plants. Noy Meir extends his argument to include herbivores stating their dependence on vegetation for food and water.


13

These species pass into resistant or dormant stages in dry seasons hence are arid-passive with a 'pulse-reserve' Hfe strategy. The autoecological view is that populations of plants and animals are 'too sparse' and 'too transient' to have much of a modifying effect on microclimate or soil. Thus he concludes 'a sufficient explanation of all phenomena of populations and arid community dynamics in such deserts may consist of the independent autecological responses of individual species to the weather, without considering interspecific interactions such as competition and predation.' Noy Meir (1973) listed three obvious attributes of arid ecosystems: (1) precipitation is so low that water is the dominant controlling factor for biological processes; (2) precipitation is highly variable throughout the year and occurs in infrequent discrete events; (3) variation in rainfall is unpredictable. Therefore he defined desert ecosystems as 'water controlled ecosystems with infrequent discrete and largely unpredictable water inputs'. Based on those considerations and definitions, Westoby and Bridges (Noy Meir, 1973) developed the pulse-reserve paradigm as a basic and general model for desert ecosystem function. In that model a rain event triggers a pulse of activity like the growth of vegetation, a variable portion of which is 'lost' to mortality and/or consumption but some part is put into a reserve such as seeds or reserve energy stores in roots or stems (Fig. 1.2). The magnitude of the pulse varies as a function of the 'trigger' event and season of the year as well as the magnitude and duration of the event.

Lost (dies or consumption)

^

— PULSE (growtti)

storage

RESERVE (seeds, roots stems)

Figure 1.2 The pulse-reserve conceptual model proposed by Noy Meir (1973) for the functioning of desert ecosystems. (From Noy Meir (1973) with permission from Annual Review of Ecology and Systematics Volume 4, by Annual Reviews.) www.AnnualReviews.org

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The question that must be asked about this paradigm is: is it of sufficiently general applicability to describe the responses of most organisms living in deserts? If it is not generally applicable, how must it be modified to adequately describe and predict organism responses?

1.7 TEMPORALLY UNCOUPLED RESOURCES The conventional wisdom concerning deserts is that water is the essential variable and that population responses and processes, such as rates of energy flow, can be understood as direct responses to rainfall. Analyses of plant production data for 4-6 successive years in several arid ecoystems has shown that the direct relationship with rainfall does not always hold (Ludwig and Flavill, 1979; Floret and Pontanier, 1982; Le Houerou, 1984). These investigators and others (Ettershank et al., 1978; West and Skujins, 1978) have suggested that available nitrogen may affect ecosystem processes during periods of adequate moisture. However, since the system components frequently behave as would be expected by the autecological hypothesis, it is logical to conclude that there are a number of important and complex linkages between precipitation and nitrogen availability. This requires a re-evaluation of the autecological hypothesis and modification of the 'pulse-reserve' paradigm. If nitrogen availability regulates composition and quantity of production, are there feedbacks through consumers and decomposers that affect nitrogen availability? In other words do deserts behave primarily as predicted by the autecological hypothesis or as predicted by the ecosystem hypothesis? If deserts behave as ecosystems, then a modified nitrogen-regulated, 'pulse-reserve' paradigm is a more appropriate model for desert ecosystems. The available evidence for these hypotheses will be evaluated in subsequent chapters.

1.8 THE SOIL RESOURCE As noted above, the pulse-reserve paradigm may not be a model with general applicability to arid ecosystems. The structure and processes of terrestrial ecosystems vary largely as a function of soil properties. The most important factor affecting the structure of vegetation in an ecosystem is soil. Soil is the basic resource of terrestrial ecosystems and arid ecosystems are no exception. Soil is a product of rock weathering. Soil formation is the result of complex interactions between lithological characteristics (geological history of parent rock), climate, and biological activity (Fig. 1.3). These interactions determine the texture and chemical composition of the soil. However, the spatial distribution of soil types in a landscape is the result of the geomorphologic history of the area. Climate variables (wind and water) redistribute soil in addition to contributing to weathering. According to the pulse-reserve paradigm, ecosystem processes in arid and semiarid ecosystems are triggered by water availability. Rainfall is a poor measure of


15

Climate

Vegetation

Figure 1.3 The interactions between soil, climate, and biota emphasizing feedbacks that affect the characteristics of each of these key ecosystem elements.

water availability. Plant growth is dependent on the ability of the plant to access soil water. The measure of water availability is not soil water content (percent of mass that is water), it is soil water potential. Soil water potential (matric potential) is a measure of the amount of force necessary to move water out of the soil matrix into a root. Soil water potential is a negative number that is a measure of the vacuum (negative pressure) necessary to extract water from a soil. Water molecules adhere to clay particles by electrostatic force. The relative proportions of sand, silt, and clay therefore determine how tightly water is held by the soil. Water potential is expressed in megapascals (MPa). Soil at field capacity (pore space saturated) has a soil water potential of approximately -0.03 MPa. Wilting point for agronomic species is generally considered to be -0.15 MPa. Desert soils may dry to water potentials of -0.6 MPa (no gravimetric water present). Textural properties determine the differences in the relative availability of water from soils that have the same water content. The supply of nutrients to plants is dependent upon water movement. The characteristics of desert soils that affect nutrient availability and nutrient cycling include pH, texture, organic matter content, and landscape position. Most desert soils are classified as aridisols, which are divided into those with an argillic (clay) horizon referred to as Argids and those with no argillic horizon, referred to as Orthids. Other soils that occur in deserts are molHsols, which are soils of high base status with a dark A horizon, Entisols which are weakly developed soils and Vertisols, the cracking clay soils. Desert soils may have argillic horizons with accumulated clays, gypsic horizons enriched with calcium sulfate more than

16


15 cm thick or calcic horizons with calcium carbonate in the form of secondary concretions more than 15 cm thick. Most desert soils are slightly basic to very basic and also tend to be well buffered by a calcium carbonate-bicarbonate system. The pH of desert soils affects phosphorus and micronutrient availability since these are less soluble at pH > 7.0. Soil texture combined with landscape position determines infiltration and movement of the wetting front. Thus, soil water availability (soil water potential) and aeration vary greatly with soil texture and landscape position because of water run-off-run-on relationships. Soil textural properties are affected by soil organic matter. Low soil organic matter content in general characterizes desert soils. However, there may be some locations within a desert watershed where soil organic matter contents are similar to soils of mesic environments. Soil texture, landscape position and organic matter content have marked effects on the population densities, species richness and biomass of the soil biota. Understanding the relationships between the soil biota and the abiotic features of the soil environment is essential if we are to understand nutrient cycling processes. Soil textural characteristics are a function of the percentage composition of sand, silt, and clay. Range of particle sizes are: sand, 2-0.05 mm; silt, 0.05-0.002 mm; clay, < 0.002 mm. Soils in the general class sandy loam to sand have high infiltration rates, rapid water percolation, and are well aerated. Loams, silty loams, sandy clay loams and clay loams, have moderate infiltration rates, moderate rates of water percolation and may have water-saturated, anaerobic microsites following heavy rains. Clay soils are sticky when wet, have low rates of infiltration and percolation and are frequently anaerobic when wet. These general characteristics of soils are important determinants of how well the autecological, pulse-reserve paradigm applies to a given landscape unit.

1.9 ECOSYSTEM PROCESSES The ecosystem processes addressed by most general ecology texts are energy flow and materials cycling. Much of the work on desert ecosystems focused on energy or trophic relationships within the system. The trophic relationship studies formed the basis for the pulse-reserve paradigm (Noy Meir, 1973) which Hnks pulses of productivity to pulses of rainfall. The pulses of productivity provide the resources for the populations of consumers and lead to reproduction in consumer populations. This paradigm is a more formal statement of the widely held view that deserts are water-limited systems and that system responses should be predictable from a knowledge of water inputs. Although material cycling (nutrient cycling) processes have been recognized as keys to understanding forest and grassland ecosystems these processes have received little attention by desert ecologists because of the prevailing view that water is the only important limiting factor in arid ecosystems. Recent work in the Sahel, Tunisia, New Mexico and Australia has indicated the critical importance of nitrogen as a factor interacting with water


17

affecting arid land productivity and trophic relationships. It is the interaction between water and critical materials like nitrogen that must be the focus of efforts to understand the dynamics of desert ecosystems and provide the basis for predicting long-term patterns of productivity and stability of such systems. These are also the critical basic studies needed for designing management schemes for arid lands that will maintain productivity and stable ecosystem structure. As discussed in the previous section, it is not sufficient to understand processes at only one point in space and time. Other processes affect the patchiness within a definable landscape unit or habitat and exchanges between landscape units. These processes affect patchiness and exchanges vary across space and over time. Processes such as herbivory which result in patches of differential removal of photosynthetic area, patches of fecal matter accumulation and patterns of wastage of plant parts by herbivores must be addressed using the basic currencies that drive the system, e.g. water and nutrients. Consumers must be considered not only as consumers of plant material but also as agents affecting the structure of the system and the fate of the currencies that operate the system. In this approach consumer organisms are viewed in part as regulators of rates as suggested by Chew (1974) and also as effectors of landscape structure and soil characteristics which impact directly on basic ecosystem processes of energy flow materials cycling and resource distribution. Biotic interactions may modify responses of species to the abiotic environment. For example, competition may be of extreme importance as a determinant of the water-nutrient availability patterns of plants, patterns of seed dispersal and patterns of herbivory that affect carbon allocation patterns and morphologies of plants. It has been suggested that competition is most important as a force organizing community structure only during 'crunch' years, that is, years when critical resources are in short supply (Weins, 1977). The modified pulse-reserve model hypothesizes that competition and predation may uncouple the critical resources threshold from direct linkage with rainfall. Thus, 'community' processes can affect lags in critical ecosystem processes and contribute to the variability of the climate-system response processes. Before proceeding with a detailed consideration of spatial and temporal variability in important ecosystem processes, it is necessary to examine the structural characteristics of desert ecosystems. Landform (geomorphology) has a direct effect on soils and catenary (watershed) relationships. Soils and catenary relationships directly affect vegetation plus water and materials redistribution. Thus understanding the interaction of the currencies (water and nutrients) requires a basic consideration of desert geomorphology. Rainfall in desert areas is highly variable and responses of desert ecosystems to rain events will vary depending on the characteristics of the particular event. Characteristics of rain events that affect ecosystem processes and landscape processes include seasonality, intensity, quantity, and frequency of recurrence. Since rainfall is the most important driving variable, these characteristics of rainfall in deserts must be considered in detail.

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1.10 PROBLEMS OF SCALING If the ecosystem is the functional unit for study of processes such as energy flow and materials cycling, the problem of scaling then becomes critical. The studies of ecosystem processes and data available in the literature generally report the results for a plot or plots ranging in size from a fraction of a square meter to several square meters. Numerous other studies focus on one species within an ecosystem and report productivity, herbivore utilization, nutrient concentrations and distribution, etc. for that species within a relatively small homogeneous area. Given that the data are largely based on small, relatively homogeneous bits of landscape, what is the procedure for and reliability of scaling up to an entire landscape or broad geographic areas. These large-scale areas contain numerous subunits, ecosystems, that differ in varying degrees with respect to component species, soil characteristics, topographic relationships etc. The scaling problem is less severe if we can discern some general rules or patterns that will allow us to incorporate the variability within a subunit, into a larger unit. In practice, scaHng problems are generally dealt with by considering small areas to be representative of larger areas and simply multiplying by the appropriate factor. Thus the data for 0.1 m^ plots are summed, averaged and that number multipHed by 100,000 to express the data as numbers ha~^ or kg ha"^ There would be no problem with scaling in that manner if the variation were uniform over what appears to be a homogeneous area. Unfortunately there may be considerable variation among patches within an apparently homogeneous area that renders that assumption false. The abihty to scale up from a knowledge of processes at the square meter scale to larger landscape units and eventually to the globe is essential. The relationships between key system processes and geomorphology are beginning to be understood. Efforts to scale up from square meters in uniform vegetation to landscapes within a region necessitate a different perspective of ecological problems. The questions have to be addressed at a different scale. We need to know how patch processes modify the exchanges of resources within a single landscape unit and what resources and populations pass through the boundary y?ter between landscape units. In order to develop the understanding necessary to manage arid lands in a sustainable manner in the future, we will need to understand not only patch dynamics but also the dynamics of inter-patch and inter-landscape unit exchanges.

REFERENCES Allen, T. F. H. and Hoekstra, T. W. (1990). The confusion between scale-defined levels and conventional levels of organization in ecology. J. Veget. Set 1, 5-12. Chew, R. M. (1974). Consumers as regulators of ecosystems: an alternative to energetics. Ohio J. Sci. 72, 359-370. DePloey, J. (1982). A stemflow equation for grasses and similar vegetation. Catena 9,139-152. Dregne, H. (1986). Desertification of arid lands. In F. El Baz and M. Hassan (eds), Physics of Desertification, pp. 4-34. Martinus Nijhoff, Dordrecht.


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Ettershank, G., Ettershank, J., Bryant, M., and Whitford, W. G. (1978). Effects of nitrogen fertilization on primary production in a Chihuahuan Desert ecosystem. J. Arid Environ. 1,135-139. Floret, C. and Pontanier, R. S. (1982). Management and modelling of primary production and water use in a south Tunisian steppe. /. Arid Environ. 5, 77-90. Le Houerou, H. N. (1984). Rain use efficiency: a unifying concept in arid-land ecology. J. Arid Environ. 7, 213-247. Ludwig, J. A. and Flavill, R (1979). Productivity patterns of Larrea in the northern Chihuahuan Desert. In E. C. Lopez, T. J. Mabry, and S. F. Tavison (eds), Larrea, pp. 130-150. Centro de Investigacion en Quimica Aplicada. Saltillo, Mexico. Ludwig, J. A. and Cornelius, J. M. (1987). Locating discontinuities along ecological gradients. Ecology 68, 448-450. May, R. M. (1976). Theoretical Ecology: Principles and Applications. Blackwell Scientific, Oxford. Meigs, P. (1953). World distribution of arid and semi-arid homoclimes. In Arid Zone Hydrology, pp. 203-210. UNESCO. Paris. Noy Meir, I. (1973) Desert ecosystems: environment and producers. Annu. Revi. Ecol. Syst. 5, 195-214. Noy Meir, I. (1979/80). Structure and function of desert ecosystems. Isr. J. Bot. 28, 1-19. Odum, E. P. (1962). Relationships between structure and function in ecosystems. Jpn. J. Ecol. 12, 108-118. Pickett, S. T. E. and Cadenasso, M. L. (1995). Landscape ecology: spatial heterogeneity in ecological systems. Science 296, 331-334. Reichle, D. E., O'Neill, R. V., and Harris, W. F. (1975). Principles of energy and material exchange in ecosystems In W. H. Van Dobben and R. H. Lowe-McConnell (eds). Report of Plenary Sessions of the First International Congress of Ecology, The Hague 1974, pp. 27-43. W. Junk, The Hague. Reynolds, J. F , Virginia, R. A., and Schlesinger, W. H. (1997). Defining functional types for models of desertification. In T. M. Smith, H. H. Shugart, and F. I. Woodward (eds). Plant Functional Types: Their Relevance to Ecosystem Properties and Global Change, pp. 195-216. Cambridge University Press. Cambridge, UK. UNESCO (1977). World Map of Arid Regions. United Nations Educational, Scientific, and Cultural Organization, Paris. Van Elewijck, L. (1989). Influence of leaf and branch slope on stemflow amount. Catena 16, 525-533. Verstraete, M. M. and Schwartz, S. A. (1991). Desertification and global change. Vegetatio 91, 3-13. Walker, B. H. and Noy Meir, I. (1982). Aspects of the stabiUty and resihence of savanna ecosystems. In B. J. Huntley and B. H. Walker (eds), Ecology of Tropical Savannas, pp. 556-590. SpringerVerlag. Berlin. Weins, J. A. (1977). On competition and variable environments. Am. Sci. 65, 590-597. West, N. E. and Skujins J. (eds) (1978). Nitrogen in Desert Ecosystems. US/IBP Synthesis Series 9. Dowden, Hutchinson and Ross. Stroudsburg, Pennsylvania.

Chapter 2

Landforms, Geomorphology, and Vegetation

In order to develop conceptual models for the relationships among units of desert landscapes, the processes of transfer among those units and processes within units, it is necessary to consider landforms and their spatial and structural relationships. The base geology/lithology of a region is an important determinant of the geomorphology and landforms that can develop in the region. The geomorphology or landforms of deserts are features which, by interacting with climate and biota, determine soil characteristics. This chapter focuses on those structural features (landforms) that affect ecosystem processes. Landforms have a direct effect on water and materials, i.e. infiltration, run-off, erosion, water storage, salt accumulation etc., and on catenary sequences of soils and nutrients. The structure of desert landscapes reflects the effects of low rainfall. Despite low rainfall, even in the driest deserts, there are productive patches. The distribution of productive patches in desert landscapes is a function of locations of limiting resource. Noy Meir (1973, 1981) suggested that if soil water was heterogeneously distributed in space, productivity would be higher than if water was evenly spread across the land surface. In order to understand water and soil nutrient distributions, it is necessary to examine the variety of topographic features of arid and semiarid environments and relate these features to the kinds of vegetation assemblages that develop on these features. In addition some topographic features are relatively stable (change little in ecological time of centuries) whereas others are more dynamic. Mabbutt (1977) groups the world deserts into two general types based on gross relief and major geologic structure: (1) shield-platform deserts and (2) mountain and basin deserts. Broad plains on granitic rocks and table lands and basin lowlands on subhorizontal platform strata characterize shield-platform deserts located in Africa, Arabia, Australia, and India. In these areas mountains or hillslopes are restricted in area to ancient mountains or, in some instances, areas to mountains of recent volcanic activity. Gently sloping erosional plains are well developed and aeolian sands, which occur as sand plains or dunes (Mabbutt, 1977), characterize the lower portions of such landscapes. In shield and platform deserts the underlying lithology and the geomorphic history of the landscape largely determines the structure and composition of the vegetation communities. For example the vegetation distribution of the Arabian shield desert of north-central Saudi Arabia correlates with six general landscape

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units that differ in geomorphologic history and Hthology (Schulz and Whitney, 1986). The rocky, high rehef areas support a tree and shrub association with an understory of bunch grasses and herbs. The surrounding sandy pediments support communities of chenopod shrubs. The ephemeral drainages (wadis) support small trees such as Acacia spp. and Lycium spp. on the benches of the main drainage channels. The extensive shield plains with fine textured soils have a vegetation that is dominated by three species of shrubs: Rantherium, Fagonia, and Astragalus. Two different shrubs dominate the sand plains: Anabasis and Hammada. The ephemeral drainages in the sand plains support a mixture of Acacia trees and Panicum grasses. The ephemeral lake margins support denser stands of the shrubs of the sand plains plus some. Artemisia shrubs. The sand seas (dunes) support a larger number of species and higher density of plants than the bedrock plains (Schulz and Whitney, 1986). Mountain and basin or basin and range deserts are characterized by mountain ranges separated by broad alluvial filled valleys. Basin and range deserts are geologically younger than the shield-platform deserts. Basin and range deserts are found in the Americas and in Asia. The alluvial processes tend to create catenary sequences with the course alluvia at the base of the mountains grading into finer sandy soils, sandy loams, clay loams and finally clays of the ephemeral lake basins. Soil saHnity may be high in sinks or ephemeral lakes and surrounding soils at the base of such landscapes.

2.1 DESERT MOUNTAINS AND HILLSLOPES Desert mountains and hills tend to be steep sided and rise abruptly from the gentle slope of the desert plains. There are few slopes of intermediate angle. The nature of hillslopes vary according to the parent materials and geological origin of the hills. Hillslopes on mountains of igneous origin tend to be characterized by debris (boulders, fractured rock) with little if any boulder accumulations at the base. Boulder fields vary in terms of boulder size distribution and physical distribution across the faces of a hill slope. In some areas boulder debris is oriented into debris chains. Boulder fields produce a variety of microsites in which plants establish. Many of the microsites receive run-on from the impervious surfaces of the rocks around the site. Even on hillslopes with an aspect facing the midday sun, microsites within boulder fields may support species of plants that require a relatively constant supply of soil water and moderate ambient temperatures. The shading and crevices in the rock and boulders provide escape sites for many species of invertebrates and vertebrates. Boulder-strewn hillslopes have received little attention from ecologists. However, the unique features of boulder fields on hillslopes which, in some instances, approach mini-oases, deserve attention (Figs 2.1-2.3). Mountains of sedimentary origin exhibit bedding with little if any boulder field formation. The debris slopes may be simple or complex depending on the


Figure 2.1

Figure 2.2 Mexico.

23

Desert mountains, the MacDonald Ranges, in central Australia.

A desert mountain with boulder-strewn slopes, the Chihuahuan Desert, New

24


:'^^, ; ^ . -^""^

r

^t,

^-vrr-Hr

Figure 2.3 An example of arid hillslopes and flat-topped hills flanking a deeply incised ephemeral stream channel in the Negev Desert, Israel.

bedded materials. In some situations rock resistant to weathering forms a cap on a hill resulting in the flat top steep-sided hills (mesas or buttes) characteristic of many arid areas. Hillslopes of sedimentary origin frequently have the form of terraced slopes. The terracing forms with the differential rates of weathering of the different sediment strata and accumulation of soil size particles on weathered benches. The terraces support an array of vascular plants. On the terraces, rainfaU IS enhanced by run-off water from the surrounding rock. The rock blocks between terraces may be largely devoid of vegetation except for cryptogams (Uchens, algae, and moss) or may support vascular plants in crevices and surface fractures! 2.2 PIEDMONTS, ALLUVIAL FANS, AND BAJADAS Piedmonts are graded slopes of alluvial materials extending outward from the base of mountains, mesas or hills. Piedmont gradients appear to be the result of interactions of coarseness of surface debris, nature of sediment load, magnitude and frequency of run-off across the surface, rate of stream channel cutting and density of perennial vegetation. These same factors plus the chemical nature of the parent rock largely determine the soils developed along piedmont gradients. The characteristic morphologies of drainages that develop on pediments tend to be dendritic moving from rills to small channels into larger channels.


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Alluvial fans form at the outlet of mountain valleys (Fig. 2.4). The concentric contours of alluvial fans are formed from alluvia deposited as stream velocity rapidly decreases when released from the confines of the valley walls. Since only part of an alluvial fan receives sediment at any time, the surface is made up of sectors of differing age (Mabbutt, 1977). Bajadas are compound or coalesced fans that form piedmont plains especially in closed basins. Slope profiles are determined by the slopes of the fans that

^(dTl ^^^^/ ^ ^ 3rPa2»^J ^ C X \^x x W Pil^lt'iiUi

rxjjOl/v^ \j|n:

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X X X \

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X X X X X X X X X

• \ i 7 ^ ^ >\el \JSr*J^f ^^ JMj^x>«{rVx x>

iiiSisE

X XJ

^(eTl \^S,^(^f^j^ v.- T ' ^ > ^ J C'K^^ ^ xL^ / Vfir ^.A * ^ «

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Figure 2.4 Alluvial fans coalescing into a piedmont bajada (from Parker, 1995). (a) Aggradation at canyon mouths, then coalescence of fans into bajadas during the early to midPleistocene, (b) Increasing stream power relative to sediment load causing fanhead entrenchment and a shift in deposition to lower parts of the fan, soil forming on stable interflu ves. (c) Deposition of materials on deposited materials (Qf2 on Qfl). (d) Reduction in sediment load relative to stream power (early Holocene) (e) Erosion by sheet wash and deposition on distal fan. (From Parker, 1995 with permission of Journal of Arid Environments, Academic Press Ltd.)

26


coalesced there by forming an undulating surface that gradually grades into slopes of finer alluvia in basin areas that may form ephemeral lakes. Alluvial fans are one of the most common landforms in intermontane desert regions. Alluvial fans are conical-shaped deposits of fluvial and debris flow sediments that accumulate where stream channels emerge from bedrock-constrained mountain valleys. As the stream channels that are confined by upland rock structure emerge onto the intermontane basins, changes in channel morphology and flow velocity reduce the energy of the stream and its ability to transport sediment. This results in deposition of materials ranging in size from boulders to coarse sand. Where streams drain adjacent valleys fans coalesce over time to form a broad piedmont or bajada. Depending on the geological history of the mountains and basin, the deposition of materials may result in a simple gradient where the sorting of particles produces an exponential decrease in downslope particle size (Graf, 1988). Most alluvial fans and bajadas are not formed by simple depositional gradients. Because of a complex geological history, depositional and erosional episodes result in complex surface deposits on what appear to be simple alluvial fans. Each depositional episode may build a different segment of the fan from sediments of different origins and particle sizes (Mabbutt, 1977). Episodic floods rework previously deposited material and, depending on the depth and energy of the flood, carry coarse materials farther downslope where they bury finer materials deposited by earlier small events. This results in complex patterns of alluvial deposits across the fans and bajadas. Episodes of relative stability create surfaces of different ages and different degrees of pedogenic development. Older soils frequently have well-developed clay-enriched argilHc horizons or petrocalcic horizons, which are absent in younger soils. The interaction of pedogenic and geomorphic processes produces a complex of spatial mosaics of soil texture rather than a simple Hnear gradient (Gile and Grossman, 1979). Mosaics or simple linear gradients may support complex vegetation associations (Parker, 1995). For example, McAuliffe (1991) found large vegetation differences among alluvial terraces with relatively simple geomorphic histories. Other studies have concluded that marked changes in vegetation composition coincide with the boundaries between geomorphic surfaces of different ages (Parker, 1995). Many investigators have suggested that plants respond to variation in water availability governed by soil texture. The coarser soils on the upper fans store run-off water thereby providing more mesic sites than the finer textured soils on the lower slopes (Fig. 2.5). This feature may be a key to the higher diversity of vascular plants observed on upper fans. However, Parker (1995) indicated that other soil physical properties may be more important than surface texture in controlling water availability. These properties include depth to an impenetrable layer, texture of the subsoil, variability in faunal activity (biopedturbation) and the areal extent of surface stone pavement which affects both infiltration and erosion by raindrop splash. The spatial distribution of various plant life forms is a function of the interaction of several environmental variables: soil texture, elevation, soil nutrients, and

Landforms, Geomorphology, and Vegetation Horizontal Scale (km)

27

Surface Elevation (m)

h 1500

h 1400

1300

Major Physiographic Part

Basin Floor

Piedmont Slope

IMajor Landform

Playa

Fan Piedmont

z

ee o Ik Q Z

2

Component Landform

Alluvial Plain Non-burled Fan Remnant

Alluvial Fan Apron

Mountain Alluvial Fan

Erosionai Fan Remnant

Fan Collar

Figure 2.5 Relationship between elevation, slope gradient, soil, and vegetation on a Chihuahuan Desert bajada (From Wondzell et al, 1987). Geomorphic surfaces range in age from Jornada 1 at 250,000-400,000 years BP to Organ II at 1100-2100 years BR

relative dryness (based on slope angle and aspect). Soil texture and nutrients may be distributed along an elevation gradient or may exhibit a mosaic pattern that is produced by the interaction of run-on, run-off, and perennial vegetation. Vegetation associations reflect the spatial pattern of these variables. For example, in the northern Sonoran Desert, the large columnar cacti exhibit different abundance patterns based on abiotic variables. The saguaro (Carnegiea gigantea) reaches maximum densities on coarse granite-derived alluvial soils of flat upper bajadas and basin floors. The organ-pipe cactus (Stenocereus thruberi) is most dense on coarse sandy soils of steep, south-facing granitic slopes. The senita {Lophocereus schottii) is restricted to very coarse, granite-derived alluvial soils along the banks of washes (ephemeral streams) (Parker, 1988). Alluvial fans and bajadas of relatively uniform age are also made complex by the movements of stream channels across the fan surfaces over time. These meandering stream channels cause a shifting mosaic of stable and unstable substrates and discontinuities in soil properties. Complex patterns of vegetation assemblages on fans and bajadas are the product of the complexities of geomorphic history and pedogenesis. Understanding ecosystem processes therefore requires

28


Figure 2.6 A desert watershed toposequence (piedmont grassland (foreground), creosotebush (Larrea tridentata) shrubland, grass-shrub mosaic and ephemeral lake basin) in the Chihuahuan Desert, New Mexico.

an understanding of the geomorphic history and soil properties in the landscape units that make up the fan or bajada. The lithology of the mountains interacting with the prevailing climatic conditions of the geological past and the ecological present are the most important factors controlling the development of vegetation on the alluvial fans and alluvial plains. In their evaluation of the processes leading to the development of plant communities on a desert watershed that developed from a granitic monzonite mountain, Wondzell et al. (1996) list ecological processes and geomorphic processes of the landforms that control the structure of the vegetation community. The percentage slope and soil characteristics determine the erosion/deposition relationships. The fan collar with slope of 7-10% is a depositional area despite the relatively steep slope because of the coarse nature of the soil, which allows high rates of infiltration. The alluvial fan components with slopes of 2 ^ % may be erosional or depositional features depending on the soil characteristics and the type of vegetation that is developed on that soil. On the lower slopes sediments are transported by sheet flow as suspended sediment and/or bedload. The bedload materials are coarse and are lost from sheetflow when the velocity is reduced. The vegetation of the fan collar is desert perennial grassland that exhibits a relatively sharp transition (ecotone) to


29

a desert shrub community (Fig. 2.6). The lower slopes of the fan were occupied by perennial desert grassland until grazing by domestic livestock during droughts opened these areas to colonization by shrubs. The current vegetation of the lower slopes is a grassland mosaic with small insular shrubland patches. The importance of lithology as a primary determinant of plant community patterns in relationship to topography has been documented by studies of a gypsum hill - valley region of Spain (Guerrero-Campo et al, 1999). The species that dominate the vegetation in this landscape are gypsophylic species. However, topographic position and aspect determine the life form distribution of the gypsophylic vegetation. Tall perennial grasses dominate the valley bottoms. Scrub vegetation dominates the hillslopes and hill tops. The spatial distribution of the plant communities of this landscape is a function of water availability, soil depth and sediment transport down the hillslopes. These vary with slope and soil characteristics. The characteristics of vegetation on Sonoran Desert bajadas has been shown to be a function of slope, soil texture and soil salinity (Phillips and MacMahon, 1978). Small trees (paloverde, Cercidium micwphyllum, ironwood, Olyneya tesota, and mesquite, Prosopis juliflora) dominate the coarse soils at the top of bajadas. The large columnar cacti, saguaro, Carnegiea gigantea are abundant on the coarse soils on the upper slopes and absent on the finer textured soils on the lower slopes. Shrubs are distributed from the top of the bajada to the lowest elevations. Upper slopes and mid-level slopes are dominated by creosotebush, Larrea tridentata, and burr sages, Franseria spp. On the bajada studied by Phillips and MacMahon (1978) the lower slope soils were somewhat saline and dominated by a chenopod shrub (Atriplex polycarpa). The fine-textured soils of the basin were quite saline (electrical conductivity (EC) between 7 and 20, whereas upper slope soils had EC in the range of 0.6-3.4). Leaf succulent shrubs (Suaeda and Sarcobatus) plus the saltbushes {Atriplex spp.) dominated the saline soils. Although the soil sequences on many alluvial piedmonts (bajadas or alluvial fans) may follow simple gradients in texture and chemistry, many desert areas have a history of climate cycles with wet periods followed by dry periods of varying duration. Historical changes in climate result in changes in the volume and energy of run-off and the erosive power of floods. These historical variations can have dramatic effects on the distribution of soils and vegetation. Erosion can cut away previously deposited alluvium, leaving calcium carbonate deposition layers exposed or buried under a shallow remnant of soil. Some soil horizons (clay-rich, argillic horizons) require tens to hundreds of thousands of years to form (McAuliffe, 1994). Clay horizons and indurated calcium carbonate horizons limit percolation of water and downward transport of nutrients. When argillic horizons form on alluvial piedmonts and are subsequently dissected by later episodes of erosion, a mosaic of patches with and without argillic horizons develops. The remnant patches of piedmont with argilUc horizons support a very different mix of plant species than the adjacent eroded areas of alluvium which lack a layer that inhibits water percolation (McAuliffe, 1994) (Fig. 2.7). McAuhffe's work clearly demonstrates the importance of understanding the complex evolution of soils.

30


A Antmima(MMea

B

C^HMîaîp.

Canadhmi miaophyUum

LMrrea tridemata Ambfosia iMoidea CmckMum nucrophylkim

100

0

Horizontal Scale (m) Figure 2.7 A typical landscape cross-section across a portion of an alluvial fan with a complex geological history in the Sonoran Desert, Arizona. Zone A consists of narrow, level areas with preserved argillic horizons; Zone B consists of erosional slopes and ridgelines from which argillic horizons have been completely truncated. Dominant perennials of each landscape position are listed above the landscape unit. (From McAuliffe, 1994. Reprinted with permission of The Ecological Society of America.)

especially in areas with high topographic variability. If we do not understand the soils, we will fail to understand the soil-plant water relationships and fail to understand how various landscape units respond to variations in climate.

2.3 EPHEMERAL STREAMS Ephemeral streams vary considerably in terms of length and breadth of stream channel, complexity of channel morphology, and the riparian plant communities that develop on channel margins. Ephemeral streams that originate in mountains and that are responsible for the development of alluvial fans and bajadas are different from those streams that drain watersheds with much lower topographic relief. Ephemeral streams originate from anastomosing rills that drain upland areas of a watershed. In areas of shallow soils on steep terrain the small branches at the origin of an ephemeral stream may be steep-sided and cut to the bedrock base. Most of the small origin stream beds are covered with granular sediments. Desert ephemeral streams (wadis or arroyos) share several features depending on the position of the drainage channel with respect to the area of watershed draining at that point. In areas where bedrock is close to the surface, ephemeral channels may have eroded channels deep into the bedrock. Here the topography of the ephemeral channel is confined on one side by a steep cliff face and the depositional-erosional benches are on one side only (Batanouny, 1973). In


31

ephemeral streams where the channels cut through alluvium, the drainage channel is bounded on both sides by one or more deposition-erosion benches. The active channel is generally sterile but may support widely scattered shrubs that are both resistant and resihent to torrential flood waters and the debris transported by flood waters. Small feeder channels are generally shallow. In semiarid deserts the benches or margins are populated by species of shrubs and grasses that are found on the upland areas that supply run-off to the channel. The larger channels develop benches because the drainage channel represents flooding by average storms and the benches are flooded only by low-frequency episodic events. In long-established ephemeral streams, benches may have been formed during the wetter conditions of the Pleistocene. The deep sediments of ephemeral stream channels are mostly sands and gravels. The sediments are characterized by exceedingly high infiltration rates. Water entering ephemeral stream channels rapidly infiltrates into the stream bed. This water is referred to as transmission loss (water that leaves the flowing mass of water by gravity). Transmission losses are sufficiently high that only the very large, intense rainstorms produce floods of sufficient depth to reach the terminus of the stream channel (Fig. 2.2). The vegetation along the origin stream beds differs from the upland drainage area vegetation only in terms of cover, not species composition. Middle segment channels are broad and distinguished by border-margins of obligate and/or facultative riparian plant species (Fig. 2.8). Middle segment channel sediments

Figure 2.8 An ephemeral stream channel in the Chihuahuan Desert, New Mexico. The channel margin vegetation includes mesquite (Prosopis glandulosa) and the obligate riparian species Brickelia lanciniata in the left foreground.

32


tend to be coarse sands. Cross-section morphology of middle segment channels may include more than one channel and several benches. The benches of ephemeral streams afford habitats that can be occupied by both ephemeral and perennial vegetation (Batanouny, 1973). In the ephemeral streams in the hyperarid region of Egypt, the perennial vegetation is sparse even on the stream terraces that are rarely inundated by flood waters (Batanouny, 1973). In the northern Chihuahuan Desert, a region receiving approximately 250 mm rainfall per year, mid-segment ephemeral stream channels are lined with dense vegetation (close to 100% canopy cover). That vegetation is composed primarily of deep-rooted species such as mesquite (Prosopis glandulosa) and desert willow {Chilopsis linearis). One of the obligate shrub species, Brickellia lanciniata, appears to be both resistant to flood damage and resilient if damaged, and will occasionally establish in the channel. Shrubs in the channel trap sediments and organic debris during floods. This results in island formation that splits the stream channel. Depending on the location of such islands with reference to the surrounding drainage area, the stream bed may become braided into several channels that coalesce into a single channel further downstream. The deep coarse sediments of the mid-segment channels produce the conditions for transmission losses. Transmission loss occurs when stream-flow water percolates into the sediments. Transmission losses account for marked reduction in flow volumes over the reaches of desert streams. For example, Mabbutt (1977) reported that a stream in the Sahara that received run-off equal to 80% of the rainfall on the upstream catchment, was reduced to 15-20% of the rainfall downstream. On a larger ephemeral stream draining a basin of nearly 35,000 km^ in India, Sharma et al. (1984) reported a 70% decrease in flow volume at a gauging station 300 km downstream. Lengths of mid-segment channels are a function of the topography and area of the watershed drained by the channel. The obligate and facultative riparian species bordering mid-segment channels probably reflect the lengths of long-term average flows. The obligate and facultative riparian species have access to the large volume of transmission loss water stored in the sediments. Transmission losses reduce the volume and energy of channel flood waters. Reduction in volume and velocity results in deposition of bedload materials and of coarser suspended sediments. The terminal-segment channels are generally depositional features. In this segment of ephemeral streams, flow velocity is greatly reduced. Lower parts of terminal segments may receive flood water from less than half of the storms that result in flow in the upper segments. Species composition of vegetation along the terminal segments is largely the same as the surrounding area. The desert riparian vegetation associated with ephemeral streams provides habitat for many animal species that are absent from the upland areas. Ephemeral stream channels are linear features that are characterized on a different spatial scale from the surrounding area. The patchiness of desert riparian vegetation probably reflects differences in water and nutrient contents of the sediments (Atchley et al, 1999; Killingbeck and Whitford, 2000).


33

2.4 BASINS AND FLATLANDS The most obvious factors affecting run-off, run-on, and type of organic matter transported by overland flow are slope and length of slope affected byflow.Areas with slopes of less than 1% experience sheet flow or overland flow during some large rainfalls. In arid regions with low slopes and withfinetextured soils, vegetation may develop in banded patterns (Mabbutt and Fanning, 1987). Banded vegetation patterns have been described in Africa, Australia, the Middle East and North America (White, 1971). The most intensively studied banded vegetation is that of patches of closely spaced trees (primarily Acac/(2 spp.) interspersed in a matrix of much sparser vegetation. In Africa, this type of vegetation is called 'brousse tigree' (tiger stripes) and other authors refer to patterned vegetation as vegetation arcs. Vegetation in banded patterns is not restricted to Acacia spp. woodlands. It has been described in chenopod shrublands (Dunkerley and Brown, 1995), desert grassland (Worrall, 1959) and in tabosa grass (Hilaria mutica), tarbush {Flourensia cemua), mesquite {Prosopis glandulosa) mosaics (Montana, 1992). Patterned vegetation divides a slope into narrow contours of vegetation, representing run-on areas or sinks, separated by barren or low vegetation cover, run-off slopes. The effect of this spatial arrangement of vegetation is to concentrate surface water into a small surface area, thereby multiplying the effective moisture by a factor determined by the infiltration rate on the run-off slope and the area of the run-off slope. Banded vegetation develops on slopes as small as 0.5° (Dunkerley and Brown, 1995). In chenopod shrublands, the topography is a stepped microrehef of approximately 10 cm. The low benches are occupied by dense vegetation. The upslope sides of the vegetation bands intercept water and transported debris from overland flow (interception zone). The dynamics of banded vegetation described in a conceptual model by Tongway and Ludwig (1990) has been verified by the empirical studies of Montana (1992). The model proposed by Tongway and Ludwig (1990) relating the grove-intergrove run-off, run-on relationships is summarized in Fig. 2.9. According to this model the downslope side of the grove and the intergrove have low infiltration rates and excess water flows across the surface transporting leaf fragments, kangaroo and sheep dung, seeds and other organic matter to the upslope side of the next grove. The accumulation of organic matter on the upslope

Sfove

Interceptisr! Ilome

grosio« Slope

Leg mmM

Figure 2.9 A schematic of the spatial relationships of mulga {Acacia aneura) groves and the intergrove barren patches or erosion slopes (Based on Tongway and Ludwig 1990)

34


side of the groves, increases the fertihty of the soils in addition to the increased water storage which enhances estabhshment of seedUng mulga trees. Thus, over long periods of time. Acacia die out on the downslope side of the groves, establish on the upslope side and the grove slowly moves up slope. In northern New South Wales grove-intergrove mulga woodlands are interspersed as plains with broad drainages with dense mulga groves at the landscape scale of tens of kilometers (Fig. 2.10). Infrequent very large rain events (e.g. 50-75 mm in 2-^ h) are thought to generate overland flows of sufficient magnitude to fill the depressions of the ancient drainage channels that developed during wetter periods in the past. These flows probably transport organic matter from the grove-intergrove subsystem into the drainage basin thus acting somewhat like a scouring event. Sand plains and floodplains are relatively flat features occupying basins in basin and mountain range topography and are common in shield-platform deserts. There is evidence that the geological history of these seemingly stable landscapes is important as a determinant of soil characteristics and the vegetation that develops on those soils (Friedel et al., 1993). In central Australia, the alluvial sand plains that had been deposited < 700 years BP and older landscape units such as red earth alluvial fans deposited in floods 1500-2000 years BP had vegetation dominated by long-lived trees and shrubs (Acacia aneura, Ventilago viminalis.

Figure 2.10 An example of banded vegetation shown from the perspective of the erosion slope looking toward the mulga (Acacia aneura) grove. Note the A. aneura skeleton on the erosion slope that forms a 'log mound' on the otherwise flat surface of the erosion slope. Photo from the Lake Mere station in northwestern New South Wales, Australia.


35

Figure 2.11 An example of a desert flatland, the Nullabor Plain, South Australia. The shrub dominant on this portion of the Nullabor Plain is bluebush, Maireana spp.

and Atalaya hemiglauca). The less stable surfaces are erosional features. The more stable surfaces exhibit erosion of only a few centimeters of topsoil. Shrubs with life spans of a few decades populate these surfaces. The erosional surfaces that are less stable have exposed subsoil and are populated by grasses and herbaceous plants (Friedel et aL, 1993). There are unstable depositional areas that are mobile sands that are moved by present day flooding and older depositional features that represent infrequent (episodic) flood deposition. The vegetation patterns on this landscape respond to similar soil-forming processes that McAuliffe (1994) documented for Sonoran Desert alluvial fans. Thus, in sand plain or flood plain landscapes, the characteristics of the landscape units vary as a function of return time of disturbance (either aeolian or fluvial) and the parent material from which the soil was formed (Fig. 2.11). Intermediate slopes (> 0.5%-5%) are characterized by rills or runnels which are small channels that do not necessarily terminate in a larger channel. These conditions are met on the lower slopes of piedmonts or bajadas. Because of the steeper slopes, run-off tends to be channeled into rills or runnels. Sheet flow from small areas (tens of square meters) transports leaf and stem fragments, small feces, and fine sediments into the rill channels. The volume and velocity of the rills are relatively low, hence small obstructions or a reduction in slope angle result in spreading of the water and organic matter in relatively narrow bands

36


(l-5m in width) across an area of generally less than 10 m in length. This creates an interrupted dendritic pattern of water flow and organic matter accumulation behind debris dams. The vegetation in the accumulation zones has generally the same species composition as the run-off zone areas and differs primarily in density and/or size of individual plants. 2.5 DESERT PAVEMENTS AND STONY SURFACES Stone strewn surfaces are common features in arid environments. They range from rock or boulder strewn surfaces to plains of gravels. Using the Arabic terms for such landscapes, areas covered with large rocks are called hamadas whereas those with a covering of smaller stones and pebbles are regs. The accepted explanation for the origin of such features is loss of fine materials by deflation until the stony surface protects the finer material below from further deflation. However, on sloping terrain stone pavement may form from below by differential erosion. In areas where stony tablelands have an underlying layer of clay stone or shale, stony gilgai relief may form. Gilgai, from the Australian aboriginal world for small waterhole, is thought to form from subsurface swelling by wet subsoils. These are characteristic landforms in the south and west of the Australian arid zone. Soils in the depressions are sandier than surrounding clays. This microrelief may be important for plant growth on such stony places. Other desert stony surface features develop from hard crusts that resist erosion, e.g. calcrete (cemented calcareous materials), gypcrete (cemented gypsum sulfates) and silcrete (silica cemented sand and gravels). These durable surfaces are of interest geologically but support Httle if any vegetation (Fig. 2.12).

Figure 2.12 An example of desert pavements and sand dunes. Foreground, stony pavements; background sand dunes-sand sea, Gobabeb, Namibia.


37

Batanouny (1973) provided an example of vegetation distribution in a stony or gravel landscape. He described the gravel surface as 'covered by closely packed gravels providing an impenetrable surface to root penetration, so it is usually sterile.' This surface is dissected by runnels (small channels) that capture windand/or water-borne materials. The type of vegetation that is supported in these channels is dependent on the depth of soil materials accumulated in the channel. The fine branches of the drainages support only ephemeral plants whereas the larger channels that result from the merging of small channels have deep soil and support perennial shrubs such as Artemisia monosperma and grasses such as Panicum turgidum.

2.6 RIVERS AND FLOODPLAINS Rivers in desert regions may be conveniently divided into those that originate in wet mountainous areas and traverse a desert, e.g. Nile (Africa), Rio Grande and Colorado (North America), and Indus (Asia) and those that originate in semiarid or arid regions such as the Lake Eyre drainages in Australia. The former are generally permanent streams whereas the latter are largely ephemeral. Perennial rivers and their flood plains, because of their high water tables are considered to be mesic ecosystems. Although there are many interesting features of such systems, they are beyond the scope of this book. Ephemeral streams in deserts tend to be wide and often trench-like. Many desert streams tend to become braided with individual channels splitting and recombining. Channels tend to have sand or fine gravel beds that are readily transported during floods as 'bedload'. Depending on the nature of the watershed, desert streams may also produce clayey flood plains as the discharge volumes vary depending upon storm characteristics in the source area. The variation in discharge and stream volume affects the degree of aggredation or degradation of various parts of stream channels. These processes in turn affect the characteristics of vegetation along stream margins and in the areas between channels.

2.7 SAND DUNES AND SAND FEATURES The features that most people associate with the term desert are sand dunes. Sand dunes are important features of the driest deserts but a less common feature of semiarid deserts. Aeolian landforms result from the interactions of wind and the land surface and are, therefore, sensitive to changes in prevaihng direction and intensity of wind. The formation of aeolian deposits and landforms require '(1) sources of sand and dust, (2) sufficient wind energy to transport these materials, and (3) conditions favoring accumulation of material in depositional areas' (Lancaster, 1994). The location and characteristics of present-day sand dunes, sand seas, and other aeolian features are products of geological history modified

38


in some instances by recent land use practices. The importance of geological history in determining the characteristics of aeolian features as habitat for plant assemblages is similar to that described for alluvial piedmonts. For example, the Kelso sand dunes in California (USA) include active surface sands and stabilized dunes. The stabilized dunes are protected from deflation and sand movement either by vegetation or by gravel lag colluvium (Lancaster, 1994). In this dune field, the active dunes are composed of finer materials than the stabilized dunes. The most important variable affecting characteristics of these aeolian features was the spatial relationship to sources of sediment. A Pleistoceneriverand lake system that gradually dried provided the source of most of the material in the dune system (Lancaster, 1994). The morphology of aeolian features is the basis of the general classification of such features. Crescent-shaped or barkhan dunes (a form of transverse dune) form by wind-blown sand grains that form mounds with slip faces on the lee side of the mound. These dunes move slowly in a downwind direction and tend to be mobile. In areas with large quantities of easily moved sand, barkhan dunes may coalesce into linkage groups of transverse dunes. Transverse dunes are the most active and tend to be migratory (Thomas, 1992). In areas with active transverse dunes, most of the vegetation is limited to the inter-dune slacks. However, because of the high infiltration rates characteristic of sand dunes, the flanks of transverse dunes may support dense populations of ephemeral plants in addition to scattered perennials. Longitudinal (linear) dunes are elongated in the same direction as the sand transporting winds. These dunes generally have broadflanksand narrow crests. In Australia and the Kalahari, the mobile sand is largely limited to the crest and vegetation stabilizes the flank slopes. There are several theories for the formation of longitudinal dunes, such as extension of barkhans, wind sift and extension downwind from points of sand supply (Mabbutt, 1977). The linear dunes of shield-platform deserts are important landscape features that have a long history of development. This history is reflected in the complex vegetation communities that develop on the dunes. The dune soils differ in terms of lithologic origin and weathering history. The soils of the Kalahari can be divided into red, pink, and white sands with more than 90% sand (0.02-2.0 mm diameter) and fine soils that are sandy loams, sandy silts, and sandy clays (Skarpe, 1986). In the western Kalahari Desert, Skarpe (1986) described 16 vegetation types. The primary factor separating these vegetation types is the textural and Hthologic origins of the sands and spatial characteristics of the dunes (height, breadth, and spacing). Sandcovered undulating flats and low dunes support open shrub savanna assemblages and/or scattered Acacia spp. and grasses. In areas with taller dunes, there are differences in species composition of the grasses on the dune crests (mostly Stipagrostis amabilis) and the dune slacks where Stipagrostis obtusa is dominant. The patterns of vegetation on individual linear dunes reflect differences in water and nutrient availability. The broader geographical differentiation of communities reflects differences in general soil types and in some areas the effects of recent anthropogenic disturbance.


39

There are similarities between the hnear dune areas of southern Africa and central Austraha. The landforms are the result of 'paleo-winds' (pevaihng winds of past geological eras such as peak of Pleistocene glaciation) and there are distinctive soil differences among the crests, flanks and slacks or swales. The dominant vegetation of the Australian dunefieldis open spinifex hummock grassland (Buckley, 1981). Spinefex (or porcupine grass) is a common name applied to a group of hummockforming grasses with tightly curled leaves that are spine-like in appearance and effect. The main species are Triodia basedowii, Triodiapungens, and Plectrachne schinzii (Buckley, 1981). The linear dunes of central Austraha are 5-25 m high and 0.2-1.5 km apart. There is a consistent soil catena on the sand ridges and a consistent floristic gradient from dune crest to swale (Buckley 1982). Most plant species are restricted to a single topographic position. Species occupying the dune crests require loose sandy soil. The species on the lower dune flanks occupy sites with clayey sand. There is a strong dune to swale gradient in total soil nitrogen. Thus even in sand dune environments, subtle differences in soil texture and nutrient status resulting from the geomorphic history of the landform must be considered in order to understand the ecosystem processes of this type of landscape. Dunes also form where vegetation traps moving sand. This is important in semiarid regions and especially important in areas where vegetation cover and composition is changing by direct or indirect effects of human use and management (Gibbens et al, 1983). Trees and shrubs may serve as the foci for dune formation such as the coppice dunes formed around mesquite, Prosopis glandulosa in the southwestern USA and northern Mexico. In areas where mesquite cover increased with a concomitant decrease in grass cover, wind erosion accounted for between 3.4 cm and 65 cm (Gibbens et al, 1983). Sand movement continues in coppice dune dominated areas resulting in deflation from interdune slacks and deposition within the coppiced shrubs. Finer materials, aeohan dust, made up of soil particles less than 0.05 mm may be deposited in areas downwind from the source and form layers. This fine material or loess may be locally important as in the Negev Desert, Israel where loess has accumulated between steps on the limestone hills and has accumulated up to 3 m thick in the valleys. In Negev Desert sand dune areas, virtually all of the intercepted water infiltrates into the sand. The areas with highest shrub biomass and largest shrubs are the vegetated sand dunes. The adjacent loess-covered hills and valleys support much smaller shrubs. This vegetation pattern probably develops in this winter rainfall desert where water availability in the early part of the growing season is most important for productivity. Water stored in the finetextured loess soils is less available (due to lower energy gradient between soil water and root water potentials). Water storage in dunes is also a function of the underlying geology. If the dunes form on a calcrete or duricrete base, water stored in the dune sand will be available to plants. If dunes form on a permeable substrate, infiltrated water may move to deep groundwater storage. Dune soils tend to have very low nutrient levels because of the rapid percolation of water through dune profiles (Abrams et al., 1997). Even the sandy

40


sediments of ephemeral stream channels may have higher nutrient levels than dune soils. In the Namib Desert, most soil nutrients are linked to landscape position and are much less related to the plant communities. 2.8 LANDFORM STABILITY As can be seen from this brief overview, the more extensive desert landforms interact with precipitation and wind to establish the particle-size gradients. Particle-size gradients interact with climate and vegetation to form the soils. Landform also affects velocity and spatial distribution of run-off water (water inputs exceeding infiltration) across the landscape. Landform with its unique pattern of soils is a product of the geological history that has shaped the landscape. This spatial-temporal context is essential for understanding patch dynamics and resource transport among patches and landscape units. REFERENCES Abrams, M. M., Jacobson, P. J., Jacobson, K. M., and Seely, M. K. (1997). Survey of soil chemical properties across a landscape in the Namib Desert. J. Arid Environ. 35, 29-38. Atchley, M. C , deSoyza, A.G., and Whitford, W. G. (1999). Arroyo water storage soil nutrients and their effects on gas exchange of shrub species Chihuahuan Desert. J. Arid Environ. 43, 21-33. Batanouny, K. H. (1973). Habitat features and vegetation of deserts and semi-deserts in Egypt. Vegetation, 181-189. Buckley, R. (1981). Soils and vegetation of central Australian sandridges. I. Introduction. Aust. J. £co/. 6,345-351. Buckley, R. (1982). Soil requirements of central Australian sandridge plants in relation to the duneswale soil catena. Aust. J. Ecol. 7, 309-313. Dunkerley, D. L. and Brown, K. J. (1995). Runoff and runon areas in a patterned chenopod shrubland, arid western New South Wales, Australia: characteristics and origin. / Arid Environ. 30, 41-55. Friedel, M. H., Pickup, G., and Nelson, D. J. (1993). The interpretation of vegetation change in a spatially and temporally diverse arid Australian landscape. J. Arid Environ. 24, 241-260. Graf, W. L. (1988). Fluvial Processes in Dryland Rivers, Springer-Verlag, New York. Gibbens, R. P., Tromble, J. M., Hennessy, J. R., and Cardenas, M. (1983). Soil movement in mesquite dunelands and former grasslands of southern New Mexico from 1933 to 1980. /. Range Mgmt. 36, 145-148. Gile, L. H. and Grossman, R. B. (1979). The Desert Project Soil Monograph: Soils and Landscapes of a Desert Region Astride the Rio Grande Valley near Las Cruces, New Mexico. US Department of Agriculture, Soil Conservation Service. Guerrero-Campo J., Alberto, R, Hodgson, J., Garcia-Ruiz, J. M., and Montserrat-Marti, G. (1999). Plant community patterns in a gypsum area of NE Spain: I. Interractions with topographic factors and soil erosion. J. Arid Environ. 4 1 , 401-410. Killingbeck, K. T. and Whitford, W. G. (2001). Nutrient resorption in shrubs growing by design and by default in Chihuahuan Desert arroyos. Oceologia 128, 351-359. Lancaster, N. (1994). Controls on aeolian activity: some new perspectives from the Kelso Dunes, Mojave Desert, California. /. Arid Environ. 27, 113-125. Mabbutt, J. A. (1977). Desert Landforms. Australian National University Press, Canberra.


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Mabbutt J. A., and Fanning, P. C. (1987). Vegetation banding in arid Western Australia. /. Arid Environ. 12,41-59. McAuliffe, J. R. (1991). Demographic shifts and plant succession along a late Holocene soil chronosequence in the Sonoran Desert of Baja California. J. Arid Environ. 20, 165-178. McAuliffe, J. R. (1994). Landscape evolution, soil formation, and ecological patterns and processes in Sonoran Desert bajadas. Ecol. Monogn 64, 111-148. Montana, C. (1992). The colonization of bare areas in two-phase mosaics of an arid ecosystem. /. £co/. 80,315-327. Noy Meir, I. (1973). Desert ecosystems: environment and producers. Annu. Rev. Ecol. Systemat. 4, 25-52. Noy Meir, I. (1981). Spatial effects in modelling of arid ecosystems. In D. Goodall and H. Perry (eds). Arid Land Ecosystems, pp. 411-432. Cambridge University Press, Cambridge. Parker, K. C. (1988). Environmental relationships and vegetation associates of columnar cacti in the northern Sonoran Desert. Vegetatio 78, 125-140. Parker, K. C. (1995). Effects of complex geomorphic history on soil and vegetation patterns on arid alluvial fans. / Arid Environ. 30, 19-39. Phillips, D. L. and MacMahon, J. A. (1978). Gradient analysis of a Sonoran desert bajada. Southwestern Naturalist 23, 669-680. Schulz, E. and Whitney, J. W. (1986). Vegetation in north-central Saudi Arabia. /. Arid Environ. 10, 175-186. Sharma, K. D., Choudhari, J. S., and Vangani, N. S. (1984). Transmission losses and quality changes along a desert stream: the Luni Basin in N. W. India. /. Arid Environ. 7, 255-262. Skarpe, C. (1986). Plant community structure in relation to grazing and environmental changes along a north-south transect in the western Kalahari. Vegetatio 68, 3-18. Thomas, D. S. G. (1992). Desert dune activity: concepts and significance. /. Arid Environ. 22, 31-38. Tongway, D. J. and Ludwig, J. A. (1990) Vegetation and soil patterning in semi-arid mulga lands of Eastern Austraha. Aust. J. Ecol. 15, 23-34. White, L. P. (1971). Vegetation stripes on sheet wash surfaces. / Ecol. 59, 615-622. Wondzell, S. M., Cunningham, G. L., and Bachelet, D. (1987). A hierarchical classification of landforms: some impUcations for understanding local and regional vegetation dynamics. In E. F. Aldon, V. Gonzalrd, E. Carlos, and W. H. Moir (eds). Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones, pp. 15-23. General Technical Report RM-150. US Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort CoUins, Colorado, USA. Wondzell, S. M., Cunningham, G. L., and Bachelet, D. (1996). Relationship between landforms, geomorphic processes and plant communities on a watershed in the northern Chihuahuan Desert. Landscape Ecol. 11, 351-362. Worrall, G. A. (1959). The Butana grass patterns. J. Soil Sci. 10, 34-53.

Chapter 3

Characterization of Desert Climates

When we think of 'deserts' we think of hot dry cHmates. Certainly hot and dry are good adjectives to describe deserts in a very general way. Operationally there are many aspects of 'hot and dry' climates that must be considered when attempting to understand the responses of plants and animals and ecosystem processes to desert climate. Bioclimatic regimes are generally determined by temporal and spatial averaging of weather patterns (Neilson, 1986). Regional climate establishes the limit for local climatic variation and determines the structure of plant communities (Neilson, 1986). Local weather variation results from these broad geographic-scale phenomena. Local weather variables drive processes such as primary production and nutrient cycling. These variables include seasonality of rainfall with respect to seasonal temperatures, storm intensity-duration relationships, frequency of precipitation events, frequency-event size relationships, return times of particular rain events, wind - season and intensity, daily patterns of microclimates, rainfall-potential evapotranspiration relationships. Episodic events, such as extended periods of drought, have long-term effects on the structure and function of desert ecosystems. Droughts may result from changes in sea surface temperatures of the world's tropical oceans (Palmer, 1986). Drought is the climatic feature of arid and semiarid regions that causes the greatest concern to human populations and to governments. Drought is 'a creeping phenomenon' (Glantz, 1987). The onset and termination of 'drought' are not easily identified because drought is viewed as rainfall that is insufficient to support primary production. Glantz (1987) quoted Ivan Tannehill of the US Weather Bureau who said 'The first rainless day in a spell of fine weather contributes as much to the drought as the last day . . . No one knows precisely how serious it will be until the last dry day has gone and the rains have come again.' Although drought may be difficult to define precisely, the periodic retreat of moist air masses from arid regions during the period when rainfall is expected has many ramifications for the native biota and for the human inhabitants and their domestic livestock. It is now apparent to global climatologists that rainfall or the lack thereof in the mid-latitudes of the planet is greatly affected by the surface temperatures of the world's tropical oceans. El Nino has become a household word in the past twenty years because of the marked effect that the surface water warming of the central Pacific has on global climates. El Nino is the periodic incursion of warm surface

44


water into the eastern equatorial Pacific off the coasts of Peru and Ecuador. El Nino is a pattern of sea surface temperatures that are part of the Southern Oscillation. The Southern Oscillation is 'the seesawing of mean pressure differences (at sea level) between the western and eastern equatorial Pacific' (Glantz, 1987). Southern Oscillation events have been correlated with droughts, floods, and other climate anomalies around the world. El Nino warming appears to have a greater effect on weather patterns in southem Africa than in eastern Africa and the western Sahel. Droughts in northern Africa and the Sahel have been correlated with global scale patterns of sea surface temperatures in the Atlantic, Pacific and Indian Oceans (Palmer, 1986). General circulation models of sea surface temperatures and rainfall in Africa have shown that warm sea surface temperatures in the Atlantic and Pacific have the effect of reducing rainfall over the western Sahel but warming in the Indian Ocean produces a slight enhancement of rainfall in this region. The results of the general circulation models show that for the eastern Sahel, sea surface temperature of the Indian Ocean is the most important factor in reducing rainfall (Palmer, 1986). The best-known characteristics of the El Nino-Southern Oscillation (ENSO) is the coincidence of drought or 'below-average' rainfall in many areas of the world at the same time (Ropelewski and Halpert, 1987). For example, droughts in India, northern China, Australia, parts of Africa and the Americas tend to occur almost simultaneously (Nicholls, 1991). This phenomenon is explained by sea surface temperatures that affect high aUitude air mass flow patterns (the 'jet streams'). Changes in jet stream flows from highly zonal (west-east) to meridonal (north-south) produce distinct types of variation in weather patterns. Changes in jet stream flow have been shown to be responsible for extreme drought and excessive rainfall in the arid southwestern US (Neilson, 1986). Recent concern with global warming has focused considerable attention on how that warming will affect the periodicity of surface sea warming in the tropical oceans and the impacts of surface sea water temperatures on global circulation patterns. The ENSO has a number of important effects on rainfall on the Australian continent. For the Australian continent, there is a tendency for droughts to coincide with anomalous warm sea surface temperatures in the east equatorial Pacific and for extensive wet periods to accompany periods of cool sea surface temperatures in the east equatorial Pacific (Nicholls, 1991). Other ENSO characteristics of interannual fluctuadons in Australian rainfall include the following: variability is very large, droughts and wet periods have time scales of about one year, spatial scale is very large (continental), phase-locked with the annual cycle, and often followed/preceeded by the opposite rainfall anomaly (Nicholls, 1991). These effects of the ENSO have important implications for the vegetation of the Australian arid region. 'Climates with the same general level of aridity can offer very different mixtures of growth opportunities, because of the patterning of rainfall in time; accordingly different growth forms are found' (Westoby, 1980). The virtual absence of succulents from the Australian arid and semiarid regions is attributed to twelve-month droughts that such plants are incapable of surviving.


45

The few species of succulents found in Australia occur in locations where run-off collects even after small rain events (Nicholls, 1991). Australia has more trees at a given level of aridity than any of the other desert areas of the world. This has been attributed to the large rainfalls characteristic of anti-ENSO periods. Trees with deep roots are able to remove deep-water stores in the soil that result from large rainfall events. These heavy rainfall events frequently persist for about one year or longer which produces conditions favorable for germination and estabhshment of tree species (Nicholls, 1991). These examples serve to emphasize the necessity of careful documentation and long-term records of climate patterns linked with ENSO events in the arid and semiarid regions of the world. By examining long-term climate records and records of ENSO and anti-ENSO events for local regions, it may be possible to modify management strategies for arid and semiarid lands to avoid loss of vegetation cover and soil erosion. Coarse annual climate statistics (e.g. long-term mean annual rainfall, long-term mean temperatures) provide afirst-orderconstraint upon the potential structure of any ecosystem. Seasonality, interannual variability and other factors produce the secondary constraints on ecosystem structure (Allen and Starr, 1982; O'Neill et al., 1986). These generalizations led to the development of a conceptual model for the distribution of biomes and vegetation communities (Neilson, 1987). That model states that the seasonal timing of rainfall is in 'resonance' with the life history characteristics of the organisms that populate the biome. In other words, the timing of temperature and rainfall events within each biome allows for the selection of specific life history types adapted to this general climatic pattern and selects against other life history types. If seasonality of weather is in synchrony with the phenology and life history characteristics of an organism, that organism will survive. If they are not in phase, the organism will not survive. These biological features were incorporated into a general theory of regional control of gamma diversity patterns. If regional climate is not overly stressful (i.e. not at the survival threshold for many organisms), the diversity of habitats in the landscape will be small. If regional climate is marginal (i.e. normal range of variation approaches or exceeds the physiological thresholds for many species) minor differences in habitat structure can place some of them outside the range of physiological tolerance. In this situation the diversity of microhabitats in the landscape will be higher than in the non-stress climatic region (Neilson and Wullstein, 1983). An example of this is when organisms change their use of habitats during drought and periods of 'average' rainfall. During periods of average rainfall, the available habitats for two species of kangaroo rats overlapped and included the entire range of habitats available in the Jornada Basin. During a drought, the suitable habitats were reduced to a much smaller subset and there was little overlap in the spatial distribution of the two kangaroo rat species (Whitford, 1976). All of the major biotic regions (biomes) and their ecotones appear to be controlled by major climate patterns. Decreasing rainfall on an east to west gradient in North America establishes the location of the transitions between short-grass

46


prairie and Great Basin Desert scrub. Further south are the southern deserts with spring and autumn dry periods and mid-summer rains (the Chihuahuan and Sonoran Deserts). West along a southern transect the summer rainfall deserts are replaced by the winter rainfall Mojave Desert. On a global scale, north-south gradients are primarily temperature controlled whereas the latitudinal gradients are rainfall controlled. Ecotones between major biomes represent thermal and water requirement thresholds for a large number of species (Neilson, 1987). The standard parameters used to delineate arid climates are the water balance parameters developed by Thomthwaite (1948). The basis for Thomthwaite's classification of climates is the moisture index, /, calculated as: I = 100 [(P/PE) - I] where P is average annual precipitation and PE is average annual potential evapotranspiration. If the moisture index / is negative, it indicates a net annual water deficit. Meigs (1953) elaborated on this scheme. He classified an extreme-arid climate as one with an average precipitation of less than 65 mm, absence of precipitation seasonality, occurrences of rainless periods of 12 consecutive months and / values greater than -57. Oberlander (1979) proposed a supplementary water balance index that accounts for shortcomings in the Thomthwaite method. Oberlander (1979) points out that the ratio between water deficit and precipitation is not a good indicator of soil moisture, since very high summer deficits can occur in winter rainfall areas with long periods of soil moisture recharge in the cool months of the year. He proposed an aridity index I^ calculated as:

where ST^^^^ is the maximum soil moisture storage and ^ ^ I N ^^ ^^^ minimum monthly deficit for the quantity of water necessary to remove the deficit in the least dry month. Oberlander's /^ index has the advantage of including two measures of soil moisture, a deficit term that by itself is not sensitive to storage and a maximum storage term. This index produces + or - values indicating those arid climates with a period of soil moisture storage (+ values) and those with moisture deficits all year (- values). Minimum water deficit and maximum soil moisture storage form a continuum because only when monthly D decreases to 0 does ST begin to appear. Areas having soil moisture storage always have months with no moisture deficit (months in which storage plus precipitation exceeds the potential evapotranspiration). Areas that have moisture storage deficit in all months always have zero soil moisture storage. The /^ index clearly shows the extreme arid desert core in North Africa grading into the less arid fringes to the north and south where winter rainfall or seasonal summer rainfalls prevail (Table 3.1). The winter rainfall areas of desert regions bordering the Arabian peninsula and the Iranian Desert are identified


47

with positive I^ values. The south Asian deserts are identified with I^ values rangingfi*om+ to - depending upon location and seasonality of precipitation (Table 3.1). The Central Asian deserts tend to have I^ values that are positive. Deserts of Southern Africa have /^ values that vary from marginally negative to extremely negative in the central Namib region. The highest intensity of aridity based on the /^ values are in the coastal deserts of Chile, Peru, and Namibia (all deserts with cold offshore ocean currents) and the eastern margin of North Africa from Libya to Somaha (Oberlander, 1979). The Oberlander index (/^) by accounting for water deficit and water storage appears to be the most useful index of aridity devised to date and may potentially be used as an index of primary productivity. The Oberlander index (/^) is far more meaningful for examining the production potential of landscape units that receive run-off than are any of the other indices. However, this index has not been applied to the limited data sets on primary production in desert regions. Table 3.1 Potential Evaporation Values and I^ Values for a Variety of Arid Regions

North Africa Fort Lamy, Chad Aswan, Egypt Luxor, Egypt EI Azizia, Libya Ghat, Libya Timbuktu, Mali Marrakech, Morocco Niamy, Niger Khartoum, Sudan Gabes, Tunisia Western Sahara

PE

^b

1746 1505 1394 1115 1404 1726 1002 1849 1828

+17.9 -667 -1700 +32.6 -46.2 -23.6 +14.1 +13.6 -38.7 +5.2 -77.8

992 1106

Southwest Asia Kabul, Afghanistan Kandahar, Afghanistan Tehran, Iran Baghdad, Iraq Beersheba, Israel Amman, Jordan Dead Sea North, Jordan Jidda, Saudi Arabia Aden, Yemen

1459 1637 1788

+58.3 +45.0 +22.6 +39.9 +35.7 +58.0 -1.0 -55.7 -192

South Asia Bikaner, India Jodhpur, India Hyderabad, Pakistan Karachi, Pakistan

1639 1518 1563 1419

-3.0 -2.3 -10.8 -11.9

730 1077

960 1271

999 889

48

Ecology of Desert Systems Table 3.1 - continued PE

h

793 830 719 884 868

-292 -2350 +1.7 -106 -15.4

Southern Africa Benguela, Angola Khomo, Botswana Upington, South Africa Swakopmund, Namibia

1402 1107 1059 732

-20.8 -6.0 -8.3 -292

Australia Bourke, New South Wales Broken Hill, New South Wales Alice Springs, Northern Territory Tennant Creek, Northern Territory Cloncurry, Queensland Port Augusta, South Australia Broome, Western Australia Kalgoorlie, Western Australia

1085 938 1162 1443 1486 1010 1544 989

+3.5 +4.6 -3.3 -7.7 -5.8 -1.3 -1.2 -0.4

South America Anofagasta, Chile Iquique, Chile Coquimbo, Chile Lima, Peru San Juan, Argentina

Data from Oberlander, 1979.

3.1 SEASONALITY Most of the desert areas of the world are characterized by a wet season (Table 3.2). Most of the world's deserts are seasonal, that is, there is a predictable 'season' of rainfall but not a predictable amount of rainfall in that season. The beginning and end of the wet season may vary by more than a month but significant rainfall that is sufficient for growth of some parts of the flora often has a probability of close to 1.0. There are profound implications of seasonality of precipitation for desert organisms. For many species of animals, breeding season must anticipate the availability of resources needed to support the brood or young. There are a few species that are opportunistic with respect to reproduction but for most species the timing of reproduction is seasonal. For example. North American desert rodents in the Mojave desert, which is primarily a winter rainfall desert, breed in the late autumn and winter with the young recruited into the population coincident with the availability of fruits of spring annuals. In the Chihuahuan Desert peak recruitment of young rodents occurs in July at the beginning of the 'predictable' wet season (Whitford, 1976). Wallwork et al (1984,1986) in experimental studies of soil acari


49

Table 3.2 Seasonality of Selected Desert Areas of the World (N, number of months of the wet season; Months, beginning and ending months of the wet season; % of total, percent of the mean annual rainfall that falls during the wet season; Annual, mean annual precipitation in mm; S - PET, potential evapotranspiration during the wet season; A - PET, mean annual potential evapotranspiration.)

Desert location

N

Months

%of total

Annual

SPET

APET

Sahara Libya North Morocco North Algeria South Algeria South Libya Chad Sudan Senegal Niger

6 6 6 2 2 2 4 3 3

Nov-Feb Oct-Mar Oct-Mar Oct-Nov Dec-Jan Aug-Sep Jun-Sep Jun-Aug Jul-Sep

95 91 78 100 100 100 94 93 92

267 239 79 8 9 22 236 23 20

350 510 61 410 180 260 221 1006 1006

870 1760 2000 3000 2900 3410 3570 3470 3470

Sahel Senegal Niger Chad Senegal

5 5 5 5

Jun-Oct May-Sep May-Sep Jun-Oct

98 95 93 95

620 529 945 557

890 930 690 930

2890 2890 2890 2680

Karoo Puffuder Sutherland

6 6

Nov-Apr Mar-Aug

76 64

98 145

1100 440

1560 1100

Kalahari Botswana Upington, South Africa

6 3

Nov-Apr Feb-Apr

84 56

293 155

1160 580

1970 1970

Namib Windhoek

6

Nov-Mar

94

364

910

1990

Arabian Baghdad Basra Ammon Jidda Tehran Damascus Afghanistan

6 6 5 2 6 6 4

Nov-Apr Nov-Apr Nov-Mar Dec-Jan Nov-Apr Sep-Feb Dec-Mar

95 95 91 82 90 90 89

145 190 278 43 292 221 163

640 750 270 250 330 550 410

2690 2160 1420 1900 1560 1680 2110

Australian Alice Springs Port Augusta Kalgoorlie

5 6 7

Nov-Mar May-Oct Feb-Aug

70 59 73

251 239 245

1120 470 510

2000 1410 1410

50

Ecology of Desert Systems Table 3.2 -

continued

Desert location

N

Months

%of total

Annual

SPET

APET

Indian Jodphur Hyderabad

4 4

Jun-Sep Jun-Sep

89 85

320 178

780 830

2490 2550

Cihuahuan (North America) El Paso, TX 3

Jul-Sep

54

221

560

1690

Nov-Mar Jul-Sep Dec-Feb Aug-Sep

87

191

1410

2330

63

93

870

2410

Sonoran (North America) Phoenix, AZ 8 Yuma, AZ

5

Monte (South America) Mendoza

6

Oct-Mar

72

197

880

1320

Atacama (South America) Antofagosto, Chile 3

Jun-Aug

92

10

130

660

Data were taken from the British Air Ministry Meteorological Office Publication - Tables of Temperature, Relative Humidity, and Precipitation for the World, London (1958).

in the Chihuahuan Desert found that most taxa timed their reproduction such that the immatures entered the population in August and September, coincident with the highest probability of moist soils and microbial activity. This pattern persisted despite addition of sufficient moisture via irrigation in May-July to duplicate the late summer conditions. They also found that providing reliable moisture in winter favored an increase of a Mojave desert species that occurred at very low density in the Chihuahuan desert. Several investigators studying the ephemeral flora have emphasized the importance of rainfall seasonality. For example, similarity in abundance of spring annuals between years was correlated with differences in September-October precipitation of the previous fall in the Mojave Desert (Bowers, 1987). In this winter rainfall desert, it has been suggested that phenological events of a number of species are triggered by rainfalls of >25 mm in the early part of the rain season (September-December (Beatley, 1974)). If the heaviest rainfall in this period is < 25 mm, but >15 mm, only scattered plants are physiologically active the following spring. If no autumn rain event approaches 25 mm, essentially all perennial plants are dormant in the following spring and ephemeral plants are absent (Beatley, 1974). These generalizations were confirmed by Bowers (1987) who concluded that the more similar the amount of precipitation in the autumn months, the more similar the relative abundance of annuals. However, the differences in species composition of


51

the flora contributing to this relative abundance was not predictable based on seasonality and size of rainfall events. Species compositional dynamics appear to be tied to other factors that affect germination of species (such as soil nutrients). Seasonality of precipitation has some important effects on vegetation. There are almost no C4 summer annuals in the Negev where rainfall seasonality is marked, i.e. probabiUty of rainfall between May and September = 0. The phenologies of desert perennials are frequently seasonal and not opportunistically linked to rainfall. An appraisal of the importance of seasonality is available from the results of rainfall supplementation studies. Rainfall supplementation by irrigation did not change the timing of flowering and fruit production in creosotebush, Larrea tridentata (Fisher et a/., 1988). In studies of mesquite, Prosopis glandulosa, in several landscape units in the Chihuahuan Desert, we found that leaf set, flowering, and fruiting were seasonal and that variation in the initiation of these phenological events was a function of landscape position (i.e. relationship to cold air drainage). Mesquite on the piedmont slopes initiated leaf set and flowering two to four weeks earlier than mesquite on the basin floor. However, the phenology of mesquite was seasonal and the timing of events did not vary among drought years and wet years. The data in Table 3.2 show that rainfall in some deserts is extremely seasonal, e.g. the extreme desert areas of Algeria and Libya where rainfall is recorded only in late autumn to early winter. The northern Sahara exhibits strong winter rainfall seasonality. The southern Sahara and Sahelian regions have predominantly summer seasonal rainfall. The Middle Eastern desert areas are also strongly seasonal, with winter precipitation accounting for more than 80% of the total annual precipitation. Winter precipitation is also characteristic of the higher rainfall areas of the Atacama desert in South America. Precipitation in the arid and semiarid regions of southern Africa is not strongly seasonal. Climatically the Karoo and Kalahari are similar to the Chihuahuan Desert with more than half the precipitation occurring in the warm summer months, but with some probability of rainfall during any month of the year. The North American and South American Deserts are not strongly seasonal. The absence of strong seasonality has implications for the species composition of the ephemeral plant communities. In deserts that are not strongly seasonal, precipitation outside the wet season may be sufficiently frequent to support a distinct flora that does not develop in seasonal deserts. For example, in the Chihuahuan Desert, the C4 ephemeral flora develops virtually every year with the summer rains (except during droughts). In this desert sufficient winter rainfall to result in development of winter-spring C3 ephemerals occurs at a frequency of about one year in four. In the Sonoran Desert near Tucson, Arizona, where rainfall is distinctly biseasonal, there is a fairly predictable winter-spring ephemeral flora and a fairly predictable summer ephemeral flora. In strongly seasonal deserts, a useful measure of relative aridity is the ratio of potential evapotranspiration (PET) of the wet season to the mean rainfall of the wet season. This ratio provides an index of moisture stress during the period when most of the primary production is likely to occur. If that ratio is less than 3.0,

52


there is potential for high productivity during the wet season. In regions where this ratio is 10.0 or greater, productivity is severely limited due to lack of water.

3.2 SPATIAL EFFECTS Although temporal variability of rainfall is emphasized by investigators working in desert environments, spatial variability is equally important. Spatial patterns of rainfall in arid environments are governed by topography and type of rainfall (frontal or convectional). Even frontal rainfall in desert areas may occur with cellular patterns. In Tunisia, a typical cell size is about 6-7 km^ (Berndtsson, 1987). In the frontal storm, winter rainfall areas of the Negev Desert of Israel, cell sizes exceeded 20 km in diameter (Nativ and Mazor, 1987). Convectional storm cells vary gready in areal extent (Fig. 3.1). Many convectional cells in the Chihuahuan Desert of southern New Mexico, USA, range in size from approximately 1 km in diameter to >10 km in diameter. Not only does storm depth vary spatially, storm intensity also varies. In a detailed analysis of storm intensities in northern Tunisia, Berndtsson and Niemczynowicz (1986) found that the spatial correlation of intense rainfall was less than 1 km. They concluded that the storm center reduction factor for reduction in intensity was 50% for an area of 20 km^. Thus, areas experiencing the same storm will receive different amounts of rainfall. These

Figure 3.1 An example of a convectional rainstorm that affects a small spatial area. Photo from the Santa Rita Experimental Range south of Tucson, Arizona.


53

examples demonstrate the importance of spatial variability in rainfall as the driving variable for ecosystem processes in arid environments. The primary factor affecting storm depth and intensity and local climate is topography. Desert mountains influence rainfall by orographic effects (rapid cooling of rising air masses) which results in increasing rainfall with increasing elevation. In Tunisia, the average annual precipitation in the Atlas mountains in the northwestern part of the country is about 1580 mm. In the flat terrain of southern Tunisia, the average annual rainfall is less than 50 mm (Berndtsson, 1987). Orographic effects are not all that dramatic but are significant. The average annual rainfall on the east side of the Organ Mountains (maximum elevation 2800 m) near Las Cruces, New Mexico is approximately 625 mm. In Las Cruces (elevation 1250 m) the average annual rainfall is 230 mm. Topography not only affects precipitation, in basin and range topography, cold air drainage from mountain slopes into the valley bottoms can have dramatic effects on the length of the growing season (Beatley, 1975). The cold air drainage and resulting reduction in growing season is a major determinant of the species composition of the vegetation. Spatial patterns in vegetation can interact with climate to affect water distribution following storms. In the Great Basin desert, where most of the precipitation is in the form of snow, the taller vegetation (Atriplex confertifolia) trapped more snow than mixed stands or pure stands of shorter Ceratoides lanata (West and Caldwell, 1983). The geographic location of mountain ranges is the most important factor causing mid-latitude aridity. The extensive dry regions of Asia (Turkestan east to the Gobi Desert of China) and the Great Basin Desert of North America are the result of orographic effect on the flow of air masses (Manabe and Broccoli, 1990). The mechanisms of mid-latitude aridity are not limited to 'rain shadow' effects of mountains but are largely the effects of mountains on the polar jet stream. The interaction of mountain ranges with the polar jet determines regions of frequent (or infrequent) passage of extratropical disturbances (Manabe and Broccoli, 1990). Spatial variability in precipitation can have dramatic effects on the larger landscape scale. One large rainfall event in northwestern New South Wales covered an area of approximately 100 km in diameter. The flush of growth resulting from that event, attracted kangaroos, birds, and other animals from surrounding areas that experience little productivity during that growing season (personal observation).

3.3 PREDICTABILITY The most frequent statement about rainfall in deserts is that it is unpredictable. It is necessary to examine that unpredictability seasonally in a 'seasonal' desert. It is also necessary to examine the characteristics of the rainfall in terms of effectiveness for various ecosystem processes and groups of biota. One common analysis that has been done is to calculate the coefficient of variation (standard deviation of annual rainfall -^ the mean annual rainfall (xlOO). Analysis of data for a series of 26 desert areas showed that there was little relationship between the coefficient

54

Ecology of Desert Systems uu -

90 -

2^ c

• •

80 70 -

•

60 -

'o

1o o

•

• •

50 -

• •

• •

40 30 -

1

1

J.

50

100

150

• • • • • 1—

200

1

250

300

Rainfall (mm)

Figure 3.2 Coefficients of variation of rainfall as a function of average annual rainfall for desert areas of the world. (Data from Low 1979.)

of variation for rainfall and the mean annual rainfall of these regions (Fig. 3.2). Regression analysis of the data in Low (1979) yielded a significant negative relationship between mean annual rainfall and the coefficient of variation (p < 0.002). However, the r^ was only 0.33 indicating that only 33% of the variation was attributable to mean annual rainfall (Fig. 3.2). Thus, although the coefficient of variation in annual rainfall may be higher in arid and semiarid regions than in wetter regions, that variability is not unique to desert regions. One finding is that the coefficient of variation is generally lower in forested and mesic grassland environments (Wagner, 1981). Conley etal. (1974) addressed the question of variability of rainfall by comparing precipitation records from Lansing, Michigan and Deming, New Mexico. They found that the interannual variation was larger in Michigan than in New Mexico, but suggested that variation approaching the physiological limits of the biota was the critical issue. This is indeed a crucial issue because it focuses directly on questions of management and sustainability. In ecosystems where precipitation inputs periodically fall below the physiological threshold limits for some proportion of the biota, loss of species can change many if not most of the ecosystem processes. If variability in rainfall periodically drops below the physiological limit thresholds, maintenance of a diverse flora and fauna is dependent on patch heterogeneity at the landscape scale (i.e. patches that provide refuges for such species). There is indirect evidence for the physiological threshold concept and its relationship to patch refugia in data from long-term studies of desert small mammal and Hzard populations (Whitford, 1976; Whitford and Creusere, 1977). Several species of small mammals and lizards appeared in the trappable population


55

in creosotebush shrubland and mixed grass-shrub mosaic habitats during a period with above average rainfall. These species disappeared when annual rainfall dropped to average or below average. Therefore, for desert regions the rainfall variability statistic of concern is the frequency of rainfall amounts below the physiological threshold for the dominant species. 3.4 INTENSITY, DURATION, FREQUENCY, AND RETURN TIME The rainfall parameters that determine the effectiveness and the landscape effects of precipitation are intensity, duration, return-time and frequency of precipitation events. David Tongway of the CSIRO Rangelands Research Centre in Australia used a rainfall classification method suggested by Jackson (1958) to examine the distribution of rain events by event size. Jackson (1958) divided rain events into the categories ranging from very large (events > 25 mm) to very small (events < 3 mm). He documented that small rainfall events can account for a large proportion of the total rainfall in an arid region. Tongway used those categories to classify and compare the long-term rainfall at Yudnapinna Station in South Australia with that at Cobar in northwestern New South Wales. Yudnapinna is in a seasonal rainfall area of Australia where winter rainfall predominates. Cobar is in the area between the summer rainfall to the north and winter rainfall to the south. However, in both of these areas, rainfall events that were too small to result in water storage in the soil for sufficient time for plant growth tended to dominate the precipitation during some years (Table 3.3). The relative effectiveness of short-duration rainfall varies greatly with season and geographical location. In semiarid regions at high latitudes, the cooler prevailing

Table 3.3 Comparison of Storm Distributions from 1939 to 1955 for the Yudnapinna Station, South Australia (mean annual precipitation, 203.4 mm yr~^; vegetation, chenopod shrubland) and the Cobar Region of Northwestern New South Wales (mean annual precipitation 379.8 mm yr~^; vegetation, eucalypt-mulga (Eucalyptus -Acacia) woodland) VH

H

M

S

L

VL

Yudnappinna Station (1939-1955) No. days cumulative 5 Mean no. days year-i 0.25

35 2.1

32 1.9

34 2.0

180 10.1

658 38.7

Cobar Region (1963-1982) No. days cumulative Mean no. days year-i

82 4.1

73 3.7

66 3.3

235 11.8

896 44.8

16 0.8

Storm type classification from Jackson (1958): VH, very heavy, > 50 mm in 2 days; H, heavy, 23-50 mm in 2 days; M, moderate, 13-23 mm in 2 days; S, significant, 7.8-13 mm in 1 day; L, light, 2.8-7.8 mm in 1 day; VL, very light, < 2.8 mm in 1 day.

56


temperatures allow small rainfall amounts to percolate into the soil and provide effective moisture for plant growth (Sala and Lauenroth, 1982,1985; Dougherty et ai, 1996). In short-grass steppe, a platyopuntia cactus (Opuntia polyacantha) is reported to respond to rain events as small as 2.5 mm 'although a 10 mm event has considerably more value than a 2.5 mm or 5 mm event' (Dougherty et ai, 1996). Button and Ben-Asher (1983) summarized data from Avdat in the southern Negev desert in Israel using duration categories. Their summaries indicate the importance of small events, i.e. < 4 mm, in both seasonal (Negev and Yudnapinna) and nonseasonal (Cobar) areas. In the Negev a rainfall as httle as 12 mm produced run-off. Rainfall intensities generally ranged between 1.5 and 30 mm min"^ for events lasting between 10 min and 720 min (Button and Ben-Asher, 1983). In the Chihuahuan Desert, we conducted a number of studies using simulated rainfall (sprinkler irrigation of experimental plots). These studies examined processes such as plant growth, decomposition rates, nitrogen mineralization and composition of the soil biotic communities when water was applied in a series of small events (irrigation with 6 mm of water at weekly intervals and irrigation with 25 mm of water every fourth week) (Fisher et al, 1988; Whitford et aL, 1988a,b). When these studies were conducted during the hot months of June through September, there were no measurable differences in the soil water potential in unirrigated and irrigated plots 8 hours after irrigation. Although small rain events are effective in stimulating the activities of soil microbiota, they are relatively ineffective as a source of water for plant growth. There is considerable variation in the proportion of the total annual precipitation for a year that is composed of storms smaller than 6 mm (Table 3.4). Data on daily rainfall from a series of rain guages at Etosha National Park, Namibia and growth of grasses showed that between 35.5% and 57.6% of the total annual rainfall was effective (Du Plessis, 1999). It is not possible to establish a minimum threshold for effective rainfall that is applicable to all arid regions. The minimum storm depth that is effective for

Table. 3.4 Distribution of Rainfalls at a Recording Station on the Chihuahuan Desert Rangeland Research Center North of Las Cruces, New Mexico, USA, in the Northern Chihuahuan Desert Number of rainfall events Year

Total (mm)

Total < 6 mm

Percent < 6 mm

>25 mm

12-25 mm

6-12 mm

3-6 mm

5 0 3 (B

I 40 E 30

20 _L.

2

4

6

8 10 12 14 16 18 2 0 22 24 Hours

Figure 3.4 Variations in daily patterns of litter water content, relative humidity at the soil surface and in the soil, and ambient temperatures at various heights and depths in reference to the soil surface (data from Whitford et al., 1981, 1982 and unpubHshed results).

60


1 80 o iS

60

3 tfi

%

40

o 20

'a>

z

20

30

40

50

60

70

T e m p e r a t u r e (""O

60 50 40 30 20 10 M J 60

J A S O Months

N — soil surface - 1 m above surface - - 1 5 cm below surface — 5 0 cm below surface

50 40 30 20 10

_!_

0200

0600

1000

1400

1800

2200

Hours Figure 3.5 Variation in ambient temperature with height above the soil surface, maximum and minimum air temperatures and maximum and minimum soil temperatures (vertical Hnes), and diurnal fluctuations in surface temperatures and below ground temperatures in mid-summer in the northern Chihuahuan Desert. (Data from Whitford and Ludwig 1973, Whitford et al. 1973; Whitford and Ettershank 1975.)


61

soil surface is the most important thermal feature of the environment. Soil surface temperatures quickly rise to 60°C or above in mid-summer (Fig. 3.5) and even in mid-winter may rise to nearly 30°C (Heatwole and Muir, 1979; Whitford and Ettershank, 1975). In deserts at higher latitudes or at elevations considerably above sea level, mid-winter temperatures may fall below 0°C at the soil surface. However, at relatively short distances above and below the soil surface the thermal environment is considerably less stressful (Figs 3.4 and 3.5). Organisms that must move across the soil surface thus limit their activity to the early morning hours and to the short period preceding sunset. The high temperatures that characterize summers in deserts exacerbate the general dryness of the atmosphere over a desert. As temperatures increase, the water-holding capacity of the air increases exponentially (Fig. 3.4). Thus at high temperatures the gradient between respiratory surfaces, internal spaces within leaves, etc. and the atmosphere is extremely steep. Rates of water movement from such surfaces to the atmosphere vary directly with the steepness of the diffusion gradient. Organisms that are immobile are forced to reduce or eliminate contact with the atmosphere in order to maintain water balance. Moving into the soil to depths > 15 cm is not the only effective means of avoiding thermal stress and the attendant hydric stress. The temperature of a shaded environment 1 m above the soil surface may be less than 40°C when soil surface temperatures are > 60°C. The temperatures within a shrub canopy are not attenuated as much as temperatures in the soil at depths > 15 cm but the environment at 0.5 m above the soil surface is considerably more benign than that closer to the soil surface. Moving away from the soil surface with reduced exposure to solar radiation reduces both the radiative and convective loads. The decrease in temperature with increasing height above the soil surface occurs rapidly because air mixing breaks up the boundary layer at the soil surface. REFERENCES Allen, T. F. J. and Starr, T. B. (1982). Hierarchy: Perspectives for Ecological Complexity. University of Chicago Press, Chicago. Beatley, J. C. (1974). Phenological events and their environmental triggers in Mojave Desert ecosystems. Ecology 55, 856-863. Beatley, J. C. (1975). CHmates and vegetation pattern across the Mojave/Great Basin Desert transition of southern Nevada. Am. Mid. Nat. 93, 53-70. Berndtsson, R. (1987). Spatial and temporal variability of rainfall and potential evaporation in Tunisia. In The Influence of Climate Change and Climatic Variability on the Hydrologic Regime and Water Resources, Proceedings of the Vancouver Symposium. lAHS Publication Number 168. Berndtsson, B. and Niemczynowicz, J. (1986). Spatial and temporal characteristics of high-intensive rainfall in northern Tunisia. J. Hydrol. 87, 285-298. Bolin, S. B. and Ward, T. J. (1987). An analysis of runoff and sediment yield from natural rainfall plots in the Chihuahuan Desert. In USDA Forest Service. Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones, pp. 196-200. Rocky Mountain Forest and Range Experiment Station. General Technical Report RM-150.

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Bowers, M. A. (1987). Precipitation and the relative abundances of desert winter annuals: a 6-year study in the northern Mohave Desert. /. Arid Environ. 12, 141-149. Button, B. J. and Ben-Asher, J. (1983). Intensity duration relationships of desert precipitation at Avdat, Israel. /. Arid Environ. 6, 1-12. Conley, W., Nichols, J. D., and Tipton, A. R. (1974). Reproductive strategies in desert rodents. In R. H. Wauer and D. H. Riskind (eds), Transactions of the Symposium on the Biological Resources of the Chihuahuan Desert Region of the United States and Mexico, pp. 193-215. National Park Service Transactions and Proceedings No. 3. Dougherty, R. L., Laurenroth, W. K., and Singh, J. S. (1996). Response of a grassland cactus to frequency and size of rainfall events in a North American shortgrass steppe. /. Ecol. 84,177-183. Du Plessis, W. P. (1999). Linear regression relationships between NDVL, vegetation and rainfall in Etosha National Park, Namibia. J. Arid Environ. 42, 235-260. Fisher, F. M., Zak, J. C , Cunningham, G. L., and Whitford, W. G. (1988). Water and nitrogen effects on growth and allocation patterns of creosotebush in the northern Chihuahuan Desert. J. Range Mgmt. 41, 387-391. Glantz, M. H. (1987). Drought in Africa. ScL Am. 256, 3 4 ^ 0 . Gunster, A. (1993). Does the timing and intensity of rain events affect resource allocation in serotinous desert plants? Acâ Oecol. 14, 153-159, Heatwole, H. and Muir, R. (1979). Thermal microclimates in the pre-Saharan steppe of Tunisia. J. Arid Environ. 2, 119-136. Jackson, E A. (1958). A Study of the Soils and Some Aspects of the Hydrology ofYudnapinna Station, South Australia. CSIRO Division of Soils, Soils and Land Use Series No. 24. Low, B. S. (1979). The predictabiUty of rain and the foraging patterns of the red kangaroo (Megaleia rufa) in central Austraha. J. Arid Environ. 2, 61-72. Manabe, S. and Broccoli, A. J. (1990). Mountains and arid climates of middle latitudes. Science 247, 192-195. Meigs, P. (1953). World distribution of arid and semi-arid holoclimes. Arid Zone Hydrology, pp. 203-210. UNESCO, Paris. Nativ, R. and Mazor, E. (1987). Rain events in an arid environment - their distribution and ionic and isotopic composition patterns: Makhtesh Ramon Basin, Israel. J. Hydrol. 89, 205-237. Neilson, R. P. (1986). High-resolution climatic analysis and southwest biogeography. Science 232, 27-34. Neilson, R. P. (1987). Biotic regionalization and climatic controls in western North America. Vegetatiolii, 135-147. Neilson, R. P. and Wullstein, L. H. (1983). Biogeography of two southwest American oaks in relation to atmospheric dynamics. J. Biogeog. 10, 275-297. Nicholls, N. (1991). The El Nino/Southern Oscillation and AustraHan vegetation. Vegetatio 91, 23-36. Oberlander, T. M. (1979) Characterization of arid elements according to combined water balance parameters. J. Arid Environ. 2, 219-241. O'Neill, R. v., DeAngehs, D. L., Waide, J. B., and Allen, T. F. H. (1986). A hierarchical concept of ecosystems. In R. M. May (ed.), Monographs in Population Biology. Princeton University Press, Princeton, NJ. Osbom, H. B. (1983). Timing and duration of high rainfall rates in the Southwestern United States. Water Resources Res. 19, 1036-1042. Palmer, T. N. (1986). Influence of the Atlantic, Pacific and Indian Oceans on Sahel rainfall. Nature 322,251-253. Ropelewski, C. F. and Halpert, M. S. (1987). Global and regional scale precipitation patterns associated with the El Nino/Southern Oscillation. Monthly Weather Rev. 115,1606-1626. Sala, O. E. and Lauenroth, W. K. (1982). Small rainfall events: an ecological role in semiarid regions. Oecologia 53, 301-304. Sala, O. E. and Lauenroth, W K. (1985) Root profiles and the ecological effect of light rainshowers in arid and semi-arid regions. Am. Midi. Nat. 114, 406-408.


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Thomthwaite, C. W. (1948). An approach toward a rational classification of climate. Geog. Rev. 38, 55-94. Wagner, F. H. (1981). Population dynamics. In D. W. Goodall and R. A. Perry (eds), Arid-land Ecosystems: Structure, Functioning and Management, vol. 2, pp. 125-168. Cambridge University Press, Cambridge. Wallwork, J. A., Kamill, B. W., and Whitford, W. G. (1984). Life styles of desert litter-dwelling microarthropods: a reappraisal based on reproductive behavior of Cryptostigmatid mites. SuidAfrikaanse Tydskrifvir Wetenskap 80, 163-169. Wallwork, J. A., MacQuitty, M., Silva, S., and Whitford, W. G. (1986). Seasonality of some Chihuahuan Desert soil oribatid mites (Acari: Cryptostigmata). J. Zool. Lond. (A) 208,403^16. West, N. E. and Caldwell, M. M. (1983). Snow as a factor in salt desert shrub vegetation patterns in Curlew Valley, Utah. Am. Midi Nat. 109, 376-379. Westoby, M. (1980). Elements of a theory of vegetation dynamics in arid rangelands. Isr. J. Bot. 28, 169-194. Whitford, W. G. (1976). Temporal fluctuations in density and diversity of desert rodent populations. /. Mammal. 57, 351-369. Whitford, W. G. and Ettershank, G. (1975). Factors affecting foraging activity in Chihuahuan Desert harvester ants. Environ. Entomol. 4, 689-696. Whitford, W. G. and Ludwig, J. A. (1973). Air temperatures, air relative humidity and solar radiation studies at the Jornada site. US/IBP Desert Biome Publication. Utah State University, Logan, UT. Whitford, W G., Ludwig, J. A., and O'Laughlin, T. (1973). Soil temperatures at 10 cm and 50 cm for the Jornada site. US/IBP Desert Biome Pubhcation. Utah Univeristy, Logan, UT. Whitford, W. G., Stinnett, K., and Anderson, J. (1988a). Decomposition of roots in a Chihuahuan Desert ecosystem. Oecologia 75, 8-11. Whitford, W. G., Stinnett, K., and Steinberger Y. (1988b). Effects of rainfall supplementation on microarthropods on decomposing roots in the Chihuahuan Desert. Pedobiologia 31,147-155.

Chapter 4

Wind and Water Processes

Visitors to arid regions are often impressed by tiie funnel clouds of 'dust-devils' with their vacuum cleaner power. Dust devils or dust whirlwinds are produced by thermals that suck up soil and plant fragments transporting these materials for various distances across the landscape. However dust-devils are only an interesting manifestation of an important and pervasive abiotic process. Wind is a characteristic environmental feature in deserts. Wind erosion is primarily a feature of arid regions and the probability of wind erosion affecting soil loss decreases exponentially as rainfall increases. Most arid lands have characteristic windy seasons during which high velocity wind 'storms' persist for hours and recur at high frequency. Some undetermined portion of the dust comes from uninhabited desert, but most desert dust storms probably originate in marginally arable semiarid areas due to erosion of surfaces no longer protected by vegetation, lag gravels, or stones.

4.1 WIND EROSION Most of the available information on wind transport focuses on soil erosion and the environmental consequences of dust storms. Dust storms result in a substantial degree of deflation and erosion, especially in the world's arid zones (Goudie, 1978). Wind erosion occurs where there are long fetches (distance across unprotected soil perpendicular to prevailing wind or downslope with respect to water flow) of unprotected soil. Tilled agriculturalfieldsare especially prone to soil loss by wind erosion. Most wind erosion research has focused on variables that affect wind transport of soil particles from agricultural fields. There are a number of variables that affect wind erosion and the generation of 'dust storms'. These include areal extent of bare soil, soil moisture, extent and type of vegetative cover, and wind speed. Most research has focused on wind tunnel studies of soil moisture, soil aggregate structure, and soil texture as factors affecting the threshold velocity for entrainment of particles in a wind stream. Skidmore (1986) provided a general functional relationship for wind erosion based on numerous studies of factors affecting the entrainment of soil particles in moving air streams. That functional relationship is expressed as:

E=f(I,K,C,L,V)

66


where E is the potential average annual soil loss per unit area, / is a soil erodibility index, ^ is a soil ridge roughness factor, C is a climatic factor, L is the unsheltered median travel distance of wind across an unvegetated patch of soil (fetch), and Vis an equivalent quantity of vegetative cover Skidmore (1986) suggested that wind erosion occurs when the shear stress exerted on the surface by the wind exceeds the ability of the surface material to resist detachment and transport. Dryness increases the susceptibility of the surface to erosion as does surface roughness, and surface cover. Soil moisture affects the adhesive properties of the soil surface resisting detachment and surface roughness and cover effectively reduce the velocity of the wind. An important feature of wind erosivity is that erosivity varies with the cube of excess velocity over the threshold velocity necessary to initiate particle movement (Breed and McCauley, 1986). On barren or sparsely vegetated areas, the fine particles are winnowed from the coarser materials which move across the surface by saltation (saltatory movement involves particles bouncing across the surface). Because some of the energy of bouncing soil particles is transferred to surface particles, the threshold velocity is effectively reduced once the saltation process has started. This accounts for erosivity varying with the cube of velocity in excess of the threshold velocity. One problem with the threshold velocity concept is that there are temporal variations in saltation activity resulting from the unsteady behavior of wind. Natural wind erosion events consist of intermittent bursts of blowing soil interspersed with periods of inactivity. The level of intermittency is governed by whether typical wind fluctuations span the gap between mean wind speed and threshold wind speed (Stout and Zobeck, 1997). The increase in flux of soil mass with distance downwind (the fetch effect) involves three mechanisms: (1) the 'avalanching' mechanism which occurs when one particle moving downwind dislodges one or more particles when it impacts with the surface; (2) an 'aerodynamic feedback' effect in which the aerodynamic roughness height is increased by saltation of particles thereby increasing the momentum flux of soil particles. These increases produce a positive feedback loop with respect to distance downwind; (3) 'soil resistance' which is primarily an expression of the change in threshold velocity with distance. Changes in threshold velocity result from inhomogeneous patchiness of soil, or progressive destruction of aggregates and crusts in the downwind direction of the fetch. Empirical studies in the Owens Dry Lake, California, support this three mechanism model (Gillette et al., 1996). One difficulty encountered in modeling wind erosion is the determination of the threshold wind speed or friction velocity at which soil movement is initiated. Winds that are sufficient to initiate soil erosion are generally turbulent and gusty. When wind speeds are averaged over longer time periods (i.e. 30-60 s), the apparent threshold is considerably lower than the true wind speed at which saltation (sand grain movement) is initiated (Stout, 1998). High-frequency sampling of wind speed and saltation activity are critical to accurately determine the true threshold of a wind-eroding surface.


67

The coarser materials that move across an unvegetated surface are the texture of dune sand. The Hghter clay and silt-sized particles are transported aloft as the 'dust'. Saltating particles break interparticulate bonds in soil aggregates and in crusts releasing additional particles for entrainment. Studies of the rates of erosion in relation to the flux of saltating grains for crusted soils of different strengths (measured by flat-ended penetrometer) found that the rate of dislodgment of surface particles decreased with increasing crust strength. Dust is release from craters formed by single impacts of saltating sand particles. The volume of material removed from impact craters is a linear function of the kinetic energy of the saltating particles in an unaggregated soil (Rice et ai, 1996). The sand moved by saltation is frequently deposited in dunes or in dune-like structures around plants or other physical objects. In undisturbed deserts, even strong winds in excess of 30-50 km h~^ will not usually generate airborne dust because of the vegetative cover (Musick et ai, 1996). Even sparse vegetation reduces wind erosion by its effect as a roughness factor that reduces velocity by generating turbulence. In a laboratory wind tunnel study using wood dowels to simulate plant stems, Van de Yen et al. (1989) developed an equation to predict soil loss as a function of wind velocity, soil type, and vegetation characteristics. They found that if any of the vegetation parameters, density (AO, height (H) and diameter (£)), were increased, soil loss decreased by the square root of that factor. Their generalized equation was of the form: SL = K (Uh - Ut)N(NDH) where SL is soil loss, K is an empirically derived constant, Uh - Ut is 3. velocity parameter specific for a soil's resistance to erosion. The more resistant the soil is to erosion, the higher Ut will become and the lower Uh - Ut becomes for a particular free-stream wind velocity. Nis the number of sticks (stems) per unit area, D is the diameter of the sticks, and H is the height of the sticks above the soil surface. The effectiveness of plants in natural ecosystems in reducing the transport capacity of wind has been the subject of very few studies. Studies of growing crop plants in wind tunnel experiments showed that soil loss was highly correlated with a plant area index and canopy cover. These plant variables reduced the transport capacity of wind. Canopy cover of 4% reduced the transport capacity of a 57 km h-i wind by 50% (Ambrust and Bilbro, 1997). A study by Van de Ven et al. (1989), emphasized the importance of even dead annual plants in stabiHzing soils in arid regions. Grazing and browsing by native or domestic herbivores that reduces height and/or density of grass clumps or sparse shrubs, will increase soil loss by wind erosion in arid regions. A variable not accounted for in this model is the porosity of the vegetation cover. Sparse multistemmed mesquite plants provide less wind resistance and less protection than a dense shrub of the same height and ground cover value. Compact desert shrubs and tussocks of grass probably provide more wind resistance and soil protection than large multistemmed shrubs that have very open canopies. In the Kalahari, removal of

68


vegetation by fire, grazing, or drought resulted in a dramatic increase in nearsurface wind velocity (up to 200%) and a decrease in shear stress velocity necessary to initiate sand movement on dune surfaces. Removal of vegetation by burning reduced areodynamic roughness by two orders of magnitude and reduced mean shear velocity by about 60% (Wiggs et al., 1994). These authors concluded that the relictual nature of the Kalahari dunes was attributable to the extensive cover by grasses and other vegetation. Movement of Kalahari dune sands could only occur immediately after devegation by fire, drought, or overgrazing (Wiggs et al, 1994). The cohesive resistance of soil to wind erosion can be affected by variables other than water. In areas with widely spaced shrubs, the presence of annual plants, even if dead, contributes to the cohesive resistance of the soil because of the fine roots and fungi that grow on the dead roots. The dead roots contribute the source of carbon required by soil microflora for their growth and reproduction. Soil microbes interact with the soil particles to form aggregates. The cohesive resistance to wind erosion also varies directly with the percentage of the soil volume that is made up of aggregates (Leys and Raupach, 1991). Chepil (1951) indicated that as the fraction of nonerodible aggregates increased, erodibility decreased. The clay fraction of the soil contributes directly to the formation of soil aggregates (Tisdall and Oades, 1982). Erodibility of soils is greatest in soils with less than 10% clay content, minimal erodibility in soils with a clay content of 20-30% and increasing erodibility in soils with clay content >30% (Chepil, 1953). Biological crusts stabilize many undisturbed desert soils. Biological soil crusts consist of cyanobacteria, green algae, lichens, and mosses. Threshold wind friction velocities may be considerably higher than the wind forces experienced at a site. Foot, vehicle and livestock traffic break up biological crusts. Areas with biological disturbed crusts are frequently exposed to wind speeds that exceed the stability thresholds of the damaged crusts (Belnap and Gillette, 1998). Because biological crust organisms are concentrated in the top 3 mm of soils, sandblasting by wind can strip these crusts from the surface. Loss of biological crusts opens sites to wind and water erosion and subsequent loss of productivity. Algal mats have been shown to stabilize sand dune areas (Pluis and Winder, 1989) and to resist destruction by saltating sand if the blow-out areas were sufficiently small. However, on large blow-out (barren) areas, algal crust development was not sufficiently fast to resist growth of the blow-outs. Disturbance that breaks up root-held soil or algal mats would therefore contribute to wind erosion. Even rain crusts which form from splash erosion (fine particles that are held together by physical forces to form a 'skin' on the soil surface) have the effect of increasing the threshold velocity. Soil piles originating from digging by animals, deposits of subsoil around ant nest entrances, and termite foraging galleries on the surfaces of plants disturb the surface crusts or place soil in locations from which it is easily detached by wind. Soil ejected by burrowing and digging animals dries rapidly and loses aggregate stability, which contributes to entrainment of this soil at low

69


wind velocities. There are no published quantitative measurements of the effects of soil disturbance by animals on wind erosion and aeolian transport of soil. In arid and semiarid regions the relatively large volumes of soil moved by vertebrates and invertebrates (Whitford and Kay, 1999; Whitford, 2000) may significantly affect soil movement. The most important variable affecting wind erosion in noncultivated desert and semi-desert areas is the fetch length of unvegetated patches. The existing wind erosion models do not effectively address this variable. Herrick (unpublished data) conducted empirical studies of threshold velocities for detachment of dyed soil particles. His data are for sand particle detachment, which provides information on the threshold fetch length for saltation. Data were collected on windy days in several landscape units at the Jornada Experimental Range in the northern Chihuahuan Desert. In a bunch grass - black grama {Bouteloua eriopoda) grassland and a burrograss (Sderopogon brevifolia) - tobosa grass (Hilaria mutica) association on fine textured soil, minimum fetch length for detachment was approximately 50 cm and threshold velocities for fetch lengths between 50 cm and 300 cm ranged between 25 and 52 km h~^ (Fig. 4.1) In mesquite coppice dune with a high degree of surface roughness due to varying heights of coppices and dunes, the minimum fetch length for saltation was approximately 500 cm. Threshold velocities ranged between 18 and 35 km h~^ (Fig. 4.1). In these 55

• 50 H f^

T

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45

black grama grassland mesquite coppice dune burrograss - tobosa grass

T

-C

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40 H

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^ ''^

o

o o

30

o o

C/5 0

25 H

o

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o

o

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20 H

15

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-l

500

1000

1500

1

1

i

2000

2500

3000

Fetch length (cm) Figure 4.1 Relationship between fetch length (diameter of unvegetated patches) and threshold detachment velocity for sand particles. (Herrick unpublished data.)

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ecosystems with different vegetation cover and height characteristics, the relationship between fetch length and threshold velocity for saltation was not apparent above some minimum value for fetch length. Reduction in canopy height and cover of vegetation and disruption of the soil surface by grazers, especially during periods of reduced rainfall, can create small erosion cells (modified from the usage of Pickup, 1985), i.e. unvegetated patches of sufficient fetch length for wind to exceed threshold velocities for a particular soil. Continued movement of soil particles across an erosion cell will bury or abraid the vegetation on the downwind side of the barren patch. Barren patches in desert landscapes are also subject to water erosion, which exacerbates the growth of the cell. Wind and water erosion combine to increase the size of the erosion cells. Eventually cells will coalesce into large barren areas unless the barren patches are colonized by vegetation. When the coalesced erosion cells pass some size threshold, they will behave as an unvegetated cultivated field with respect to wind erosion. Many of the present desert landscapes have undoubtedly developed as a result of wind erosion on erosion cells that rapidly increased in size and coalesced as a result of grazing during periods of drought. In terms of ecosystem function, aeolian transport of organic matter may be of greater importance than transport of soil. Soil losses resulting from disturbance are reduced over time as stabilization by vegetation or as lag gravels, and stones form a desert pavement. However, dead plant materials are susceptible to redistribution by wind and water. The redistribution of organic materials contributes to the development of the well-known 'islands of fertility' around desert shrubs (deSoyza et al., 1997). Organic matter accumulations under shrubs are related to the morphologies of the shrubs, shrub densities and productivity of the surrounding areas. Because of the small size and mass, much of the organic debris, the threshold velocity to dislodge and entrain leaves, stems and fruits is quite low. Applying aerodynamic theory to the shape and mass of creosotebush, Larrea tridentata leaves, stems and fruits, deSoyza et al. (1997) calculated threshold velocities of 1.1 m s"^ (3.96 km h"^) for leaves, 1.72 m s~^ (6.2 km h~^) for fruits, and for a 25 mm stem segment 1.1 m s~^ (3.96 km h~^). Once entrained in a wind stream, leaves, small stems, etc. are carried until turbulence and eddies reduce the wind velocity and the material is deposited. Turbulence develops as a result of resistance to flow by objects in the air stream path. In sparse shrubland, plants of different morphologies differ greatly in their effectiveness as litter 'precipitators' (Fig. 4.2). Shrubs with a hemispherical shape accumulate more wind-borne litter than shrubs shaped like inverted cones. As shown in Fig. 4.2, based on aerodynamics theory, the streamline flow over and around a hemispherical shrub will cause a small local turbulence under the canopy, which will result in fragment deposition but not entrainment of that material. When that air flow pattern is compared to a conical-shaped shrub, the downflow will accelerate the flow of air around the base of the cone entraining litter in that flow. Although this analysis was based on idealized leaf, stem, and fruit shapes and shrubs reduced to simple geometrical


Hemispherical shrub Turbulence at leeward side of shrub enhances deposition of litter >^

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upper flow

^ *^Î_^—^ây ^^ 10 km h"^ were recorded on an average of 302 days yr~^ at the Jornada Experimental Range in southern New Mexico. Wind transport of seeds accounts for the distribution patterns recorded by Reichman (1984) (Fig. 5.1). In desert landscapes there are microsites with high seed densities within microhabitats of low seed density. In the Sonoran Desert, seed densities were lowest in an ephemeral stream channel and in open areas between shrubs. Seed densities were not correlated with shrub volumes (Reichman, 1984). In the Mojave Desert, seeds accumulated around shrubs and the densities of seeds were correlated with the volumes of the shrubs (Nelson and Chew, 1977). Differences in seed densities in relation to shrub size in the Mojave and Sonoran Deserts are probably the result of differences in patch characteristics in these two deserts. In the Mojave Desert, rodent burrows are primarily associated with shrubs (deSoyza et ah, 1997; Soholt and Irwin, 1976). In the Sonoran Desert and in the Chihuahuan Desert surface pits produced by rodents are relatively common and exhibit clumped distribution (Steinberger and Whitford, 1983). Rodent-produced pits are important as seed

6,4001}

5,600€

Figure 5.1 Estimated densities of seed (number m"^) in various microsites in the Sonoran Desert. The prevailing wind is from right to left, a, rodent cache pit; b,windward side of small shrub; c, subshrub; d, rill channel; e, grass tussock. (Modified from Reichmann, 1984.) With permission of Journal ofBiogeography, Blackwell Science Ltd.

Patch - Mosaic Dynamics

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traps and as favorable germination sites. In the Great Basin Desert virtually all of the patches of Indian rice-grass, Oryzopsis hymenoides, occurred as a result of seeds trapped with litter in rodent excavations (Longland, 1995). Heteromyid rodents enhance the germination of the O. hymenoides seeds buried in caches by removing the seed-investing structures (lemma and palea) on approximately onethird of the seeds placed in cache pits (McAdoo et ai, 1983). In desert grassland, the spatial pattern of the vegetation was patchy but species associations within the patches were weak (Henderson et aL, 1988). The absence of strong positive or negative species association was considered to be consistent with the conclusions of Grime (1979) that competition and dominance are reduced in unpredictable desert environments. However, it is likely that this generaUzation may be restricted to arid grasslands and savannas and not to all desert environments. The high correlation between the seed bank and vegetation were hypothesized to be the consequence of frequent and unpredictable disturbance (especially variation in rainfall) and by the effects of seed consumers and the spatial patterns of their foraging behavior on seed bank turnover rates (Henderson et aL, 1988). Patchiness is typical in desert plant communities where safe sites for seeds, and germination are interspersed among more abundant unfavorable sites for seeds to escape seed feeding animals and to encounter conditions suitable for germination. Subcanopy microsites may be safe and favorable germination-establishment sites for annual plants and for some perennial plants (Whitford et aL, 1995a; DeSoyza et aL, 1997). Shrub patch development in south Texas semiarid savanna occurred in part by colonization by mesquite (Prosopis glandulosa) followed by establishment of other shrubs beneath the mesquite. The presence of mesquite shrubs attracted birds that dispersed the seeds of the shrub, Celtis pallida. C. pallida exhibited increased germination and establishment under mesquite canopies where shade reduced soil temperatures to levels that favored germination and growth. The absence of C. pallida in the patch interspaces was attributed to lack of dispersal by birds and inhibition of germination by high temperatures (FrancoPizana et aL, 1996). The patchy distribution of ephemeral plants has frequently been attributed to enhanced germination and establishment in the soils modified by shrubs and by the moderate subcanopy microclimate. The subcanopy soils and microclimate associated with shrubs has a positive effect on many, but not all, ephemeral plant species (Gutierrez and Whitford, 1987; Tielborger and Kadmon, 1995). In a demographic study of four desert annual plant species, three of the species exhibited higher emergence densities and higher seedling mortality under shrub canopies. One species (Ifloga spicata) had higher densities in open spaces. The subcanopy surviving plants produced more seeds than conspecifics growing in the openings. Ifloga spicata growing in open spaces produced more seeds than individuals of this species growing under shrub canopies. The emergence densities of plants germinating below shrub canopies also varied with the species of shrub (Tielborger and Kadmon, 1995). This provides support for the general observation that the structure of the perennial plant community affects the composition and abundance of the ephemeral plant community.

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Animal-produced pits are collection points for seeds and organic matter. The decomposing organic matter provides nutrients and increases water-holding capacity of the microsite. There are several types of animal-produced pits that serve as collection areas for seeds, litter, and dung. These include cache pits and foraging pits produced by small granivorous mammals, bedding sites of large mammals Hke the Nubian Ibex, North American jackrabbits, and Australian kangaroos (Fig. 5.2) and pits dug by porcupines (Whitford and Kay, 1999). Animal-produced pits are more important as germination sites in some locations than in others. In inter-dune pans, areas with fine-textured soils, 96% of all seedlings found were growing in diggings. In the South African Karoo, several mammals (aardvark, Orycteropus afer, porcupine, Hystrix austro-africanae, bateared fox, Otocyon megalotis and Cape fox, Vulpes chama) excavated pits that ranged in size from 7 to 24 cm long, 3 to 12 cm wide. Three types of pits were identified; (1) shallow, basin-shaped scoops made by aardvarks while 'prospecting' for termites; (2) narrow, trench-like holes dug by foxes to extract beetle and other insect larvae; and (3) circular or conical holes dug by rodents to extract insect larvae or seeds. Pits in which seeds germinated were deeper (6.7 cm) than pits without seedlings (4.5 cm). Deeper pits collect more run-off water and accumulate more plant debris and litter which contributes to the differences in germination (Dean and Milton, 1991).

Figure 5.2 Hip holes dug by kangaroos, Macropus spp., in a mulga, Acacia aneura, grove in northwestern New South Wales, Australia.


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Pits dug by some species are concentrated in areas where establishment of plants is essential to the functioning of a larger unit of landscape. For example, in the banded mulga system in AustraHa, pits produced by goannas (varanid lizards) accumulate plant debris, some fecal material and seeds of several species of plants that are important components of the interception zone of the mulga system (Whitford, 1998) (Fig. 5.3). Reichman (1984) emphasized the importance of surface disturbance as sites for seed accumulation. Although shrub canopies may generate seed accumulations, they are probably not as important as patch generators as the soil disturbances made by animals (Fig. 5.4). Another factor that has to be considered in seed dispersal and location of safe germination sites is animal dispersal. Many desert shrubs and small trees produce relatively large fruits that are dependent on dispersal by animals. Birds are effective dispersers of fleshy-fruited plant species. In a study of vegetation patch dynamics in the Kalahari, plant species with fleshy fruits occurred in 8% of the treeless plots, 17% of the plots centered on Acacia erioloba saphngs and 90% of the plots under large trees. The distribution of fleshy-fruited plants was attributed to roosting birds and mammals that utilized the shade of the large trees (Dean et al., 1999). Plants selected as preferential perch sites by birds serve as focal species for development of patches composed of bird-dispersed seeds. In the Chihuahuan Desert, the cactus wren selects tall plants such as aborescent Opuntia

Figure 5.3 Pit dug by a goanna lizard, Varanus gouldii, in the excavation of a groundnesting spider, northwestern New South Wales, Australia.

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Figure 5.4 Recently excavated cache pits produced by kangaroo rats, Dipodomys spp., in the northern Chihuahuan Desert, New Mexico.

Spp. and Yucca elata as nest sites. Cactus wren nests are constructed of grasses, many of which retain viable seeds in the seed heads. Abandoned nests fall to the ground providing a mulch and seed source in the patch where the nest was constructed (Milton et al, 1998). This suggests that patches, which have as focal species, plants selected as nest or perch sites by birds, may be sites where grasses selected for nest materials may establish albeit late in patch development. 5.2 SINGLE SPECIES PATCHES The significance of environmental modification by a single plant interfacing with the physical environment was initially described as the development of 'fertile islands' by Garcia-Moya and McKell (1970). Their studies showed that the desert shrubs modify the soil by accumulating nutrients in the subcanopy soils. Subsequent studies in warm deserts cold deserts and a variety of other arid and semiarid ecosystems corroborated these observations (Charley and West, 1975; Barth and Klemmedson, 1982; Crawford and Gosz, 1982; Virginia and Jarrell, 1983). However the 'fertile island' that develops in the vicinity of a single shrub species is not limited to nutrient concentration. Indeed nutrient concentrations are the product of a whole series of microenvironmental modifications resulting from


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the presence of a plant. As described above, a shrub, grass clump or tussock can serve as a seed and Utter trap. The accumulation of organic material within or under the canopy enriches the soil carbon stores and, if sufficiently thick, provides a habitat for a variety of soil organisms (Santos et al, 1978). The soil environment under a single grass plant is modified primarily by the addition of materials from root death and decomposition and from root exudates. Shading directly affects microclimate. The presence of a canopy reduces raindrop kinetic energy and the throughfall and stemflow water is retained within the patch.

5.3 INTRASPECIFIC MORPHOLOGICAL VARIATION The characteristics of single-plant patches vary by species and by growth form of a single-species that exhibits morphological variation. A single-species patch may provide a significant obstruction to overland flow or no effective obstruction depending on growth form. Single-species patches may or may not be effective as wind breaks. Creosotebush (L. tridentata) exhibits considerable variation in morphology. Creosotebush shrubs with exterior stem angles >45° (conical-shaped shrubs) trap no litter beneath the canopy and there are essentially no differences in subcanopy soils and intershrub space soils in terms of texture, organic carbon content and concentrations of mineral nutrients. In contrast, shrubs of this species with a growth form approaching hemispherical, accumulate subcanopy litter layers. Subcanopy soils of these plants have nutrient concentrations higher than the intershrub soils forming islands of fertility. Conical shrubs transfer a larger proportion of the stemflow to the root crown than do hemispherical shrubs (De Soyza et aL, 1997) (Fig. 5.5). These differences in subcanopy environment are reflected in production differences and species richness of annual plant species growing in the subcanopy space (DeSoyza et a/., 1997). Some of the important desert legumes (Prosopis spp.. Acacia spp.) exhibit considerable variation in growth form - single-stemmed small trees or multi-stemmed shrubs. The range of morphological variation in the dominant species of plants in desert communities has not been adequately documented. However, this example based on creosotebush, L. tridentata, from North American deserts is one example of how intraspecific morphological variation contributes to differences in patch characterisitics of single species patches. The morphology of shrubs also affects the energy of drip-line drops that may be responsible for the splash erosion contribution of mounds under shrubs. The protection provided by shrubs to the soil surface beneath the canopy results in differential rain-splash. More sediment is splashed into the area under the canopy than is splashed outward. The surface materials of the mounds under shrubs have the same size distribution as the sediments. Some of the mounding is attributed to soil loss from the intershrub space. Desert pavement (surface covering of small stones and rock) in intershrub spaces is formed of lag materials remaining after off-site transport of soil by rain-splash erosion. The microtopography and

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Ecology of Desert Systems Conical Shrub All external stems > 45 ° from the horizontal

Hemispherical Shrub

External stem angles are < 45°from horizontal

Figure 5.5 Diagram of two shrub morphologies and the effects of that mophology on stemflow and throughfall.

distribution of soil surface materials of plant patches and bare spaces is thought to be the consequence of establishment of shrub vegetation during the past century (Parsons et a/., 1992) 5.4 MOSAICS AND MULTI-SPECIES PATCHES The species composition and spatial relationships of the species are primary determinants of how the mosaic or patch functions in the landscape. Species composition may not be very predictable. For example, in a species-rich area of the southern Chihuahuan Desert dominated by a small tree, Acacia schajfneri and a cactus, Opuntia streptacantha, every combination of all common species within the association was found in the mosaics. However, the mosaics did have a common structural pattern: a perennial legume at the center, an erect (shrubby or aborescent) form of platyopuntia (flat cladodes) near or under the legume, and a decumbent form of platyopuntia at the periphery (Yeaton and Manzanares, 1986). It was hypothesized that these patches or mosaics developed following the


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establishment of a legume that modified the soil nutrient base. The colonization by aborescent platyopuntia occurs soon after the establishment of the legume. It was also suggested that O. strepacantha eventually replaces the aborescent legume, which accounts for some of the variability recorded in the structure of the mosaic. This study provides evidence that patch-mosaic structures are dynamic within the spatial and temporal constraints of the patch. Patch structure is not dependent on the temporal and spatial scales of the component species. In contrast, a study in another part of the Chihuahuan Desert found a range of patches from species poor to species rich. All of the patches were composed of species distributed around focal or 'nurse' plant species. Single species patches of creosotebush shrubs, L. tridentata, were the most species poor. Creosotebush also occurred in two species patches with cacti (Opuntia spp.) and another shrub (Jatropha dioica). Some species, mostly cacti, tended to occur in the largest patches. Sixteen species associations were positive and nine were negative. All but one of the negative associations involved one or the other of the two dominant plants, i.e. L. tridentata and Opuntia rastera. Neither of these exhibited any significant positive association with any species in the complex (Silvertown and Wilson, 1994). This led to a conclusion similar to that of Yeaton and Manzanares (1986) that focal species modify the environment allowing colonization by other species. Some species, including shrubs, such as Prosopis and Foquieria, occurred more frequently than expected on medium-sized patches. Other species, especially cacti and leaf succulents such as Yucca spp. and Agave spp. occurred primarily on large patches. There was a significant spatial structure associated with six often species examined in this study (Silvertown and Wilson, 1994) all of which occurred in clumped patches more frequently than expected. These studies suggest that the vegetation associations of patches and mosaics depend on the presence of a large focal species or 'nurse' plant for initial arrival (seed capture), establishment (favorable microsite), and survival. The bare zones between vegetation patches are taken as evidence of this dependence. The presence of a focal plant therefore operates a switch that converts local bare desert into vegetation (Wilson and Agnew, 1992). Desert vegetation is commonly dominated by plants that live for decades, centuries and even millenia (Vasek, 1980). Because of the longevity of dominant plant species, the underlying mechanisms of the dynamics of desert vegetation are difficult to study. McAuliffe (1988) provided data to support a Markovian Dynamics model of Sonoran Desert plant communities. He determined that establishment of the large shrub, L. tridentata, was predominately beneath the dense canopies of the small shrub Ambrosia dumosa. The Ambrosia shrubs eventually die and the patch is dominated by a large Larrea. Ambrosia is capable of colonizing bare (open) patches whereas Larrea requires Ambrosia as a nurse plant for its establishment. Creosotebush, L. tridentata, patches provide nurse plant environmental modification necessary for the establishment of large long-lived palo verdes (Cercidium spp.) and saguaros, Carnegiea gigantea. These large plants eventually dominate the patches in which they establish (Fig. 5.6). This results in

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Figure 5.6 Diagram showing the principal transitions in the Larrea-Ambrosia communities in the southern Mojave Desert. (From McAuHffe, 1988.) With permission by the University of Chicago Press.

Sonoran Desert ecosystems consisting of four types of patches: Ambrosia shrubs, Ambrosia shrubs with small Larrea, dead Ambrosia shrubs with large Larrea, Larrea shrubs, and open, bare patches. In the Markovian model, McAuliffe assigns transition probabilities to each of the patch states. The transitions from one patch type to another may take decades or centuries (especially in the transition of the long-lived Larrea to bare, open patches.

5.5 COMPETITION VERSUS FACILITATION Conventional wisdom, based on statements by influential desert biologists, questions the importance of and the existence of competition among plants in arid and semiarid regions (Shreve, 1931; Grime, 1979). Most of the early evidence for competition among desert plants was based on spatial distribution patterns (Fowler, 1986). The problem with spatial distribution as evidence for or against competition is that a regular distribution of plants may be the result of competition but the absence of a regular distribution is not evidence for the absence of competition. In a review of the literature on spacing in desert plants Fowler (1986) Hsts 33 studies of which nine found clumped distributions (contagious), 10 in which distributions were either clumped or random, and only two that were regular. Fowler (1986) also listed 16 studies in which there were differences in size of individuals that were related to distances between pairs of individuals (an


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indirect measure of competition). Studies of spacing of desert plants combined with information on size distributions of the plants can provide some insights into competition and its influence on patch development. In studies of vegetation of the Mojave and Sonoran Desert, small shrubs tended to be clumped, medium-size shrubs tended to be randomly distributed and large shrubs tended toward a regular dispersion pattern (Phillips and MacMahon, 1981). This pattern was interpreted as evidence of the increasing importance of competition as plants grow. Root systems were sufficiently extensive to overlap or abut in the interplant open spaces. The results were not dependent upon the position of a site on the large climatic gradient across the winter-rainfall Mojave to the bi-seasonal rainfall of the Sonoran Desert (Phillips and MacMahon, 1981). Studies of direct effects of competition involve removing one or more species from a patch and monitoring the responses of the remaining species. Studies on the Santa Rita Experimental Range south of Tucson, Arizona, showed that removal of mesquite led to an increase in annual and perennial grasses (Cable and Tschirley, 1961). Removal of sagebrush, Artemisia tridentata, led to increased biomass of several species of native grasses (Robertson, 1947). Treatment of creosotebushes with a herbicide resulted in a rapid and marked increase in bush muhly, a grass that utilizes L. tridentata, as a nurse plant (Whitford et al, 1978). These early experimental manipulations provided evidence for competition among desert plants but provided no insights into the mechanisms of that competition. These insights came from studies of two Mojave Desert shrubs (creosotebush (L. tridentata) and burr sage (Ambrosia dumosa)). Water-related interference competition occurred when water availability was low. However, competition was more intense between species than between individuals of the same species (Fonteyn and Mahall, 1981). This research suggested that regular distribution of plants may be due to past competition not to present competition. This generalization is particularly applicable to long-lived perennials such as creosotebush. Not all close spatial associations of plants are negative. As suggested in the development of patches and mosaics, focal or nurse plants are frequently required for patches to develop. The nurse plant concept was developed to explain the establishment of saguaro cacti, which are largely limited to areas under canopies of trees or shrubs (Shreve, 1931; Steenberg and Lowe, 1977) (Figs 5.7 and 5.8). Nurse plants may protect young plants from excessive heat or cold or from mechanical injury and predation. Nurse plant protection is necessary not only for large columnar cacti, but it is important for a number of smaller cacti and also for some grasses (Franco and Nobel, 1989; Livingston et aL, 1997). Cactus species apparently differ in their need for nurse plants. About 89% of the saguaro seedlings were found in sheltered microhabitats under canopies of burr sage (Ambrosia deltoidea) and palo verde (Cercidium microphyllum). Nearly 30% of the seedlings of Ferocactus acanthodes were in unsheltered microhabitats where maximum soil surface temperatures reached 71°C. The sheltered seedlings ofF. acanthodes were under canopies of a bunch grass, Hilaria rigida (Franco and

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Figure 5.7 Saguaro, Carnegia gigantea, columnar cacti associated with creosotebush, Larrea tridentata, in the Sonoran Desert, Arizona.

Figure 5.8 Saguaro, Carnegia gigantea, columnar cactus associated with ironwood tree, Olynea tesota, in the Sonoran Desert, Arizona.


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Nobel, 1989). This kind of commensal symbiosis can shift to a negative interaction (competition) when the nursed plants achieve a certain size. The classical nurse plant relationship between saguaros and palo verde changes into a relationship in which the palo verde is negatively affected by the presence of the cactus. Both stem dieback and mortality occurred in nurse trees. Stem dieback accelerated the local loss of individual nurse trees (McAuliffe, 1984a). We have observed more dead stems of creosotebush with large understory bush muhly {Mulhenbergia porterii) grass plants than in shrubs with no understory perennial grass plants. The response of this grass to the removal of creosotebush suggests interspecific competition, most probably for water (Whitford et ai, 1978). Nurse plant-commensal relationships are not limited to large cacti. Other species of cacti such as Echinocereus spp. may receive protection and microclimate modification by other species of cacti, Opuntiafulgida (McAuliffe, 1984b). Nurse plant associations may also protect the 'protege' species from browsing by herbivores. This has been suggested for Mammalaria spp. growing under various shrub species in Baja California (Cody, 1993). Protection from browsing or microclimate modification may account for the relationship between creosotebush, L. tridentata, and pencil cholla, Opuntia leptocaulis. In that relationship the pencil cholla eventually outcompetes the creosotebush which when it dies, exposes the cactus to the effects of soil erosion on the shallow root system and excavation and consumption of roots by rabbits and rodents (Yeaton, 1978). Intermediate size cacti such as the cylindropuntia or chollas of the Mojave Desert have also been shown to be associated with nurse plants (Cody, 1993). Young individuals of three cholla species were associated with nurse plants, especially the perennial grass, Hilaria rigida. Adult chollas also served as nurse plants for young chollas. However, no shrubs were found to serve as nurse plants for these clindropuntias. The nurse plant association is not obligate for these species because the frequency of cholla growing in the open was approximately the same as those growing in association with other species or itself. As earlier studies documented, maturing cholla cacti compete with nurse plants for water and nutrients which frequently results in the demise of the nurse plant. Cacti and stem succulents are not the only plants that benefit from nurse plant associations. In the Sonoran Desert, ironwood tree, Olneya tesota, modify the habitat providing nurse plant benefits to columnar cacti, and large shrubs with berry type fruits (Suzan et ai, 1996). Nurse plants may serve to trap seeds, modify the microclimate of germination sites and protect seedlings from predators (Cody, 1993). These associations contribute to the variation in species composition and structure in patches in hot deserts. The facilitation of growth of species associated with nurse plants may change to a competitive relationship over time. Nurse plant associations are reported for arid environments but appear to be rare or absent in more mesic environments. The functional explanation for this observation is attributed to differences in physiological responses of the species in the association. In arid environments, facilitation occurs when improvement of plant water relations

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under the canopy exceed the costs produced by lower Ught levels (Holmgren et al, 1997). Sparse canopies of desert shrubs and the over-riding importance of water as a limiting factor tips the balance in favor of facilitation and nurse plant status. In more mesic environments, canopy densities reduce light sufficiently that competition rather than facilitation is observed.

5.6 ANIMAL-PRODUCED PATCHES Patchiness in deserts is not limited to spatial patterns of plants in relationship to soils and topography. Patchiness is also a function of animal effects on soil properties and the spatial distribution of soil disturbances (biopedturbations) on the landscape. Patchiness caused by disturbance is important in the development and maintenance of spatial and temporal heterogeneity in arid and semiarid ecosystems (Whitford and Kay, 1999; Whitford, 2000). The agents of soil disturbance that create patches in arid environments include mammals, reptiles, and a variety of arthropods. Because soil materials are transported from deep soil layers to the surface, animal disturbance patches are areas of active pedogenesis. It has been suggested that, in deserts, digging by small mammals may be the most important mechanism for 'pumping' soluble nutrients to the soil surface and may be the only mechanism for bringing insoluble materials to the surface for weathering (Abaturov, 1972). However, many species of ants and termites construct burrow systems to greater depths in the soil than mammals, and many of these transport insoluble and soluble material from more than 200 cm deep in the soil profile. The combined effect of invertebrate and vertebrate burrowing activities can produce ejected materials that cover more than 20% of the soil surface area in a single season (Whitford and Kay, 1999). The importance of animal activity in generating soil patches is probably best illustrated by the example from the banded mulga (Acacia aneura) landscapes in Australia. In these landscapes vegetation is concentrated in stripes that are perpendicular to the slope of the landscape. The large patches of virtually barren ground between stripes are erosion slopes that shed water during intense rains. Scattered across the erosion slopes are scattered logs of dead mulga trees. These logs are perched on perceptible mounds that are riddled with termite galleries and ant nest chambers. The mounds are probably formed from the foraging gallery sheets of termites. The mound material has the same textural composition as termite gallery sheeting and has lower bulk density and higher sand content than soils of the erosion slope. The macropores resulting from the galleries and chambers of the ants and termites are responsible for the higher infiltration rates recorded on log mounds in comparison with rates on adjacent unmodified soils (Tongway et al, 1989). The activities of the arthropods also contribute to the higher nutrient levels in the soils of the log mounds. The log mounds represent scattered productive patches on the barren erosion slopes. In this system, termites and ants establish positive feedback loops where increased soil moisture produces


111

more suitable conditions for the insects thereby reinforcing the water-nutrient enrichment of the patch. Some of the most impressive patches of soil disturbance are produced by mammals or by the combined efforts of mammals and arthropods. Among the largest soil patches in desert landscapes are heuweltjies (pronounced haer -vulkies) which are large soil mounds found in southern Africa. These large mounds are produced by mole rats and termites and cover areas of up to 800 cm^. The largest mounds are produced by hairy-nosed wombats {Lasiorhinus latifrons) (Fig. 5.9). These mounds can cover up to several hundred square meters and are visible on LANDSAT satelUte imagery of Australia (Loffler and Margules, 1980). Other large soil disturbance features produced by mammals include the mounds of the now extinct burrowing bettong (Bettongia lesuer) in Australia (Noble, 1993) and mima like mounds in Argentina (Ojeda et al, 1996). Not all soil disturbances produced by mammals cover large areas. Soil disturbances vary from cache pits and foraging digs (10-15 cm^) to mounds produced by fossorial mammals (30-50 cm^) and the complex burrow systems of colonial rodents (50-2000 cm^) (Whitford and Kay, 1999). Despite large variations in size, the pedturbations produced by mammals result in a number of changes in soil properties and vegetation responses (Table 5.1). In a review of the literature on mammalian pedturbations, the probable importance of landscape position, soil properties, and geomorphological history as a

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Mound system of a hairy-nosed wombat, Lasiorhinus latifrons, in South

112


Table 5.1 Effects of Mammalian Pedturbation on Soil Properties and on Vegetation in the Area or Disturbance and/or on the Mounds of Soil Ejected from the Burrows and Digs (modified from Whitford and Kay, 1999) Soil property Litter and seed accumulation Fecal pellet accumulation Subsoil to surface < Bulk density >N

Disturbance type

Vegetation response

Cache pits/foraging pits Nubian ibex bedding sites Crested porcupine digs Gemsbok resting form Aardvark, bat-eared fox. Cape porcupine foraging digs

> Annual plants > Annual plants > Density geophytes > Density annuals > Density geophytes > Density annual forbs < Perennial grasses > Biomass annuals < Cover perennials A Species annuals > Grass biomass, > Artemisia tridentata biomass

Burrow system, Dipodomys spectabilis Burrows Spermophilis spp.

< Soil moisture > Infiltration

Burrow system - desert gerbils Rodent burrows

> Total soluble salts, >CaC03 > K, >P, >SOM

Abandoned Gerbillus colonies Burrow ejecta mounds Ejecta mounds pocket gopher

<SOM, < N >N, >SOM No change soil properties

Ejecta mounds, mole rats Ejecta mounds mole rats Ejecta mounds fossorial system

>SOM, >N >N, K, Ca, Mg, Zn, Mn, B, Fe,Al

Mound Dipodomys spectabilis Heuweltjies Giant kangaroo rat system Fossorial system Ctenomys australis Mima-like mounds, Argentina Gerbil colonies Red viscacha rat mounds Bathyergid mole rats

< Native annual grass > Biomass two shrubs > Grass production > Biomass production > Biomass production A Species composition A Species diversity Establish geophytes > Pioneer species, > exotic Mesembryanthemum > Biomass annuals A Species composition > Productivity > Exotic annuals > Biomass, > species Richness > Exotic tree < Grass production Succulent halophytes < Plant biomass, A community structure

, greater than; A, change in; SOM, soil organic matter. variables affecting the magnitude and direction of changes resulting from disturbance became more apparent. Most of the conflicting results reported in the literature probably resulted from the failure to account for these temporal and spatial variables. Despite these inconsistencies, it is obvious that animal-produced patchiness is an important feature of desert ecosystems.


113

Most studies of the effects of ants on soil nutrients have focused on single locations (Lobry deBruyn and Conacher, 1990). The published data indicate that some species of ants affect soil chemistry, whereas other species do not. Ants that transport fruits and seeds, plant parts, and dead insects to the nest concentrate organic matter in the vicinity of that nest. Ant species that have an effect on soil chemistry are characterized by long-lived colonies (>10 years) and by milling seeds or processing plant parts at the nest. Long-lived colonies of Pogonomyrmex barbatus and P. rugosus mill seeds within the nest and deposit chaff in midden piles that are usually located within a meter of the nest entrance (Wagner et al, 1997). P. rugosus was reported to discard 60% of the total energy content of materials collected by foragers and returned to the nests. Soil nutrients that are concentrated in the soils of long-lived Pogonomyrmex spp. include N, P, and K (Whitford, 1988; Whitford and DiMarco, 1995; Wagner et aL, 1997). However, the effect of harverster ant nests on soils and vegetation cannot be generalized for an entire watershed or landscape. Large differences in soil nutrients were found around ant nests in mid-slope locations on a watershed but no significant differences in soil nutrients around nests and away from nests in locations at the base of the watershed (Whitford and DiMarco, 1995). It is likely that the relative effects of ants on soils vary with landscape position and the nutrient status of the soils on a particular landscape unit. Arthropods indirectly affect increased plant production both by their activities as modifiers of soil fertility and water infiltration and by their effects as consumers. Ants and termites affect both the species composition and biomass production of soils that are modified by their activities (Table 5.2). Ants that enrich the soil in the immediate vicinity of their nests include seed harvesting ants, leaf-cutting (Attine) ants, and species that transport plant and animal material to the nests. Some species of ants create chaff-trash piles around the nest perimeter that increase the organic matter content of the soil under the ejected material (Wagner et aL, 1997). Nests of an ant, Formica perpilosa, that constructs nests under small leguminous trees (P glandulosa dinA Acacia constricta), was found to enhance seed production by 1.9 times in relation to trees without nests. The increased seed production was attributed to deep wetting of soil as a result of bulk flow along nest gallery tunnels (Wagner, 1997). A corona around the base of epigeic termitaria is generally characterized by higher nutrient concentrations and soil organic matter than soils not associated with mounds (Lee and Wood, 1971a, b; Coventry etai, 1988; Lobry deBruyn and Conacher, 1995). Nutrient enrichment and modified water infiltration and storage affect both the species composition and the productivity (biomass production) of the vegetation in the vicinity of ant nests (Table 5.2). However, the limited data on the effects of termites and ants on plant biomass production and species composition must be interpreted with caution. The species of plants that increase in biomass and that are favored by the insect-modified soils are not necessarily the same for the same arthropod species on different landscape units within a region (Whitford and DiMarco, 1995). There are also inter-annual differences in the

114

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60°C but at 42 cm, where spadefoot toads have been recovered from their burrows, temperatures ranged from 23.5 to 24.8°C (Ruibal et a/., 1969). Toads emerge with summer rains, breed in ephemeral ponds and feed heavily and rapidly during the brief periods of moist soils and high humidity. This is necessary in order to store sufficient energy and nutrients for the next dormant period. While below ground, their energy metabolism drops to approximately one-fifth that of surface-active toads, urea accumulates in the blood, and plasma electrolytes accumulate (Ruibal et al, 1969). The critical stages in the life cycle of desert amphibians are the larval or tadpole stages and recently transformed stage (toadlet). The water temperatures in ephemeral ponds must be between 16°C and 33°C for normal development of most desert-inhabiting frogs in North America (Zwiefel, 1968). The eggs of some Scaphiopus spp. continue normal development to 35°C. Desert ephemeral ponds often dry quickly and the shallow remnants reach lethal levels for tadpoles. Selective advantage accrues to tadpoles with higher growth rates and large size at metamorphosis. Differential growth rates are increased by some individuals that develop a homy beak in place of the filter feeding mouth parts characteristic of most tadpoles. Tadpoles with the homy beak mouth parts are cannibalistic on their filter-feeding brethren. The cannibals grow rapidly, metamorphose quickly and become large toadlets (Creusere and Whitford, 1977). The survival of toadlets in their burrows for the first year is a function of body mass and fat storage at the time the toadlets are forced to dig into the soil for the long dry hibemation period. Juvenile spadefoot toads tolerated loss of 16% of their initial body water and 40% of their total fat reserves. Average total body lipid content of the juvenile toads dropped from 3.2% to 2.0% in six months. Only 50% of the Scaphiopus (hammondi) multiplicatus survived for six months in their burrow (Meltzer and Whitford, 1976). These data show that the ability to burrow into the soil to avoid environmental extremes must be accompanied by sufficient water and fat stores. One characteristic of desert anurans is their sudden appearance after the first heavy rain of summer. Dimmitt and Ruibal (1980) reported that toads remain in the 20-90 cm deep 'winter' burrows then move to 'summer burrows' 2-10 cm deep.

Adaptations

129

North American spadefoot toads emerge in small numbers on nights with light rain of insufficient quantity to generate run-off and flooding of the ephemeral breeding ponds. However, the toads in the shallow burrows are sensitive to sound and emerge in great numbers in response to heavy rainfalls (Dimmitt and Ruibal, 1980). 6.1.4 Reptiles Snakes and lizards are common inhabitants of all of the worlds hot deserts. Most of these reptiles maintain body temperatures close to 38°C by behavioral thermoregulation. The most common thermoregulatory behavior of lizards and snakes is movement between shade and sun allowing heat gain or loss by radiation and conduction. Typical behavior of lizards immediately after emergence from their night-time retreat, is to select a location where the animal can maximize energy gain by radiation. Lizards select large rocks, branches or trunks of trees or open ground as basking sites. When their body temperatures reach a preferred temperature, these animals move between shade and sun to maintain body temperatures within 1-2°C of their preferred body temperature (Huey et al, 1977). Desert lizards store fats in visceral fat bodies or in their tails (e.g. geckos). These fat stores thus do not retard heat flux by radiation, conduction, or convection in animals that are dependent on behavioral thermoregulation. In sand dune environments where shade-giving plants are absent some species of Uzards and snakes have skin flaps that cover the nares and lizards have fringes on the toes that allow them to 'swim' into the sand to avoid over-heating. Most lizards and snakes seek out burrows or rock crevices where the microclimate is moderate for part of each day and for extended stays during extremely dry or cold periods. The avoidance of 'stress' by reptiles in desert environments because of their efficient thermoregulatory behavior means that few if any reptiles experience thermal stress. However, the avoidance of stress of dehydration when prey become scarce is problematic. For example, during a prolonged hot, dry summer when virtually no surface foraging was observed in seed-harvesting ants {Pogonomyrmex spp.), we found several emaciated, and desiccated homed lizards {Phrynosoma cornutum) (Whitford, unpublished field notes). We concluded that the lizards died from a combination of inadequate energy intake and most importantly an inadequate supply of preformed water to replace water lost through evaporation. 6.1.5 Mammals Most small desert mammals live in burrows of their own construction and most are nocturnal. The burrows may be simple tunnels at the base of shrubs or other plants or elaborate systems of tunnels such as the burrows of the banner-tailed kangaroo rat (Kay and Whitford, 1978). The air temperature in burrows and burrow systems is similar to but not equal to that of the surrounding soil. However, at depths >15 cm, the ambient temperatures in burrows remains at 35°C or less even when soil surface temperatures exceed 60°C. Air relative

130


humidities in burrows are generally near saturation. In deep burrow systems there may be an accumulation of carbon dioxide in the burrow atmosphere. The amount of carbon dioxide accumulation is a function of soil moisture and temperature and the effectiveness of wind in burrow ventilation (Kay and Whitford, 1978). Most but not all small desert mammals are nocturnal thereby avoiding the low humidities and high temperatures of daylight hours in summer. Diurnal species limit their activity to the early morning and late afternoon hours in mid-summer (crepuscular activity cycle). However, some diurnal species of desert mammals utilize behavioral means to extend their foraging period. In the Cape ground squirrel, Xerus inauris, the tail is raised and curled over the back to shade the body. The tail of this species is large, dorsoventrally flattened, and nearly as long as the body of the animal (Fig. 6.2). During the hotter part of the day the squirrels orient themselves to maximize the area of the back that is shaded by the tail. The shade provided by the tail reduces the surface temperature of the animal by more than 5°C which allows the squirrels to expand their foraging hours (Bennett et aL, 1984). The Cape ground squirrel is able to forage continuously on the surface for up to 7 hours but would be limited to approximately 3 hours of surface foraging without tail shading. Intermediate size mammals such as rabbits, hares, jackals, foxes, etc. may construct burrows or enlarge burrows constructed by other species or may simply seek refuge in the shade of shrubs or small trees. Some species, such as jackrabbits, retreat to burrows only when air temperatures are extremely hot (> 42°C) and then remain in the burrows for short periods during the hottest part of the day (Costa et aL, 1976). Except in the hottest part of the range of the species, jackrabbits are nonburrowers. Lepus spp. generally use shallow forms at the base of shrubs as their only shelter. In the shaded form, jackrabbits lose

Figure 6.2 The use of a parasol tail by the Cape ground squirrel Xerus inauris. (From Bennett et al., 1984.) With permission by The University of Chicago Press.

Adaptations

131

heat by radiation from the highly vascularized ears to the shaded shrub structures and by conduction to the soil of the form. 6.1.6 Birds Behavioral avoidance of desiccating conditions and thermal loads is the primary adaptation of birds that inhabit deserts. Most species of breeding birds select sheltered nest sites or locations in tall vegetation where ambient temperatures > 38°C are rarely encountered. Ground-nesting species select shaded sites. Foraging activities are generally confined to early morning hours or early evening when ambient temperatures are lower and air relative humidities higher than during mid-day. Most desert birds obtain sufficient moisture from their diet to meet or exceed water lost to evaporation, urine and feces, and thermoregulatory panting (Dawson, 1984). In extremely dry and unpredictable deserts birds may exhibit nomadism. A nomad is a bird that moves from place to place without regard to season or direction (Davies, 1984). Nomadic birds may have to move long distances once the resources needed to support life are depleted in an area. Although it is not known how nomadic birds find rich resource patches, nomadic species are successful in the dry center of Austraha and in other extremely arid environments. Not all individuals of a species move at the same time, orient in the same direction or terminate in the same place. Nomadic populations obviously face high mortahty rates but the advantages to that part of the population that locates new resource rich patches must outweigh these mortality losses. However, nomadism is a characteristic of only a fraction of the bird species inhabiting a desert area. Many other species obtain all the Hfe support resources required from the same area (Davies, 1984) 6.1.7 Arthropods Most desert arthropods exhibit some kind of avoidance behavior. Some like the desert cockroaches, Arenivaga spp. escape harsh surface conditions by burrowing 20-60 cm into the sand (Hawke and Farley, 1973). At night these insects move to the surface. The timing of that activity rhythm may be triggered by the level of the temperature inversion in the soil (Cloudsley-Thompson, 1975). In hot deserts the majority of arthropods, including many desert beetles are nocturnal. In cool coastal deserts most arthropod species are diurnal (Cloudsley-Thompson, 1991). Arthropods such as myriopods (millipedes) that lack effective morphological and physiological water conservation adaptations remain in burrows or other humid microsites until rains increase surface humidity. Even species of myriopods that have morphological and physiological adaptations exhibit thermoregulatory behavior (Wooten et a/., 1975). Wooten et al. (1975) reported that at mid-afternoon millipedes ceased feeding and moved to cooler microsites within shrubs or moved into mammal burrows at the base of the shrubs. In mid-summer, the inactivity period in mid-afternoon began when shrub temperatures first exceeded 34°C at around 1400 h. Millipedes resumed activity when

132


the cool shrub microsites dropped to 32°C (Fig. 6.3). Peak feeding and activity on the surface occurred immediately after dawn and in the early evening when ambient temperatures were less than 30°C (Fig. 6.3). Desert centipedes, Scolopendra spp., which are common ground-dwelling predators in North American deserts are nocturnal and spend dayhght hours in holes in the soil, under rocks or large debris. These animals also exhibit an endogenous activity rhythm (Cloudsley-Thompson, 1975). Species of insects that live in or on vegetation move up and down in the canopy throughout the day to take advantage of moderate microclimates. This behavior has been reported in grasshoppers and probably occurs in many species of plant bugs, plant hoppers, etc. that are common inhabitants of desert perennial plants.

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Figure 6.3 Diagram of the diel behavioral patterns of the giant desert millipede, Orthoperus ornata. Diagrams on the left depict the spatial locations and activity status of the population of milHpedes during the day. Elongate millipedes are active, coiled millipedes are inactive. Diagrams (from left to right) represent a burrow, a shrub canopy, and bunch grasses. (Modified from Wooten et a/., 1975 with permission by Academic Press Inc.)

Adaptations

133

Various kinds of beetles are among the more common arthropods encountered in deserts. Tenebrionid beetles exhibit diurnal patterns of activity and seasonal activity patterns (Smith and Whitford, 1976). Some species are nocturnal and others Hmit activity to early morning and evening hours (Cloudsley-Thompson, 1975). Most desert arachnids live in burrows in the soil, e.g. wolf spiders, scorpions, whip scorpions, sun-spiders (solfugids), tarantulas, and most are nocturnal. Many desert arachnids also exhibit seasonal activity patterns. Perhaps the most extreme seasonality is exhibited by giant velvet mites, Dinothrombium spp. which emerge from their burrows in soil after heavy rains, feed voraciously, breed and return to their burrows (Tevis and Newell, 1962). Among the most abundant arthropods in all hot deserts are the social insects, termites and ants. These insects avoid extreme conditions by living in below-ground nests which may have chambers extending 2-3 m below the soil surface. Termites and ants limit their surface activity to non-stressful times of the day (Whitford and Ettershank, 1975). Subterranean termites frequently limit surface activity to periods following rains (Johnson and Whitford, 1975). Species of liquid-feeding ants that continue foraging at mid-day remain above the soil surface in the vegetation when soil surface temperatures are above lethal Hmits (Shaffer and Whitford, 1981). Subterranean termites use thermal shadows in the soil as cues to locate large items of potential food during periods when hot, dry conditions curtail surface activity (Ettershank et al., 1980). By feeding in large objects like dung pats and Yucca sp. logs, subterranean termites avoid the harsh conditions. Subterranean termites forage in litter accumulations at the soil surface in the early morning and evening when temperatures are below 30°C and the litter is moist. Based on field experiments, the threshold soil moisture for surface foraging by subterranean termites {Gnathamitermes tubiformans) is -5.4 ± 0.2 MPa (MacKay et al, 1986). 6.2 PHYSIOLOGICAL AND MORPHOLOGICAL ADAPTATIONS As with behavioral characteristics there are suites of morphological and physiological characteristics that contribute to the 'biological success' of species inhabiting deserts. Each successful species shares some adaptive characteristics with a number of other species. Successful desert-inhabiting species may or may not have some unique adaptive characteristics. Physiological adaptations that are widely shared traits of groups of desert organisms have been documented. Suites of adaptive physiological, morphological and behavioral characteristics of some desert-dwelling taxa have been extensively studied. 6.2.1 Perennial Plants Unlike animals that can escape extreme temperatures and extremely low humidity by moving to moderate microclimate sites, perennial plants have no way to escape environmental extremes. Virtually the only avoidance mechanism available

134


to perennials is to shed leaves during drought (drought deciduous species). However, producing new leaves is a water and energy intensive process. In addition drought deciduous plants are disadvantaged during the period immediately following a soil wetting rainfall because the developing leaf surfaces are very small and the plants cannot maximize photosynthesis when the required resources are most abundant. Most desert plants are not drought deciduous. However, there are a number of morphological and physiological characteristics of desert plants that allow them to survive extreme conditions and to respond rapidly during periods when water and nutrients are available. In most deserts of the world, the vegetation is a mix of C3, C^ and CAM photosynthetic pathway plants. These photosynthetic pathways differ with respect to the intermediate molecule to which CO2 is added. In the C3 or Calvin Cycle photosynthetic pathway, carbon dioxide is added to a five-carbon compound (ribulose bisphosphate) to form two molecules of a three-carbon compound (phosphoglyceraldehyde). In the C^ photosynthetic pathway, CO2 is combined with phosphoenolpyruvate, a three-carbon molecule, to form malate or oxaloacetic acid (four-carbon molecules). In C4 plants carbon dioxide is concentrated in bundle sheath cells which allow the plant to bind carbon dioxide at lower concentrations in the cell. This reduces water loss in hot environments. The CAM (crassulacean acid metabolism) photosynthetic pathway is a modification of the C^ photosynthetic pathway in which the assimilation and the light reaction of the Calvin Cycle are segregated temporally. In CAM plants carbon dioxide uptake occurs at night and the carbon dioxide is assimilated into a four-carbon compound (either malate or oxalacetate) which is stored in the leaves in high concentrations. During the daylight hours, the carbon dioxide is released from the stored four-carbon compounds and enters the normal Calvin Cycle (Fig. 6.4). The significance of different photosynthetic pathways in the adaptation of perennial plants to life in extreme desert environments is still debated In some deserts, such as the hot deserts of North and South America, one or more species of C3 plants dominate the plant communities. According to the general physiological characteristics of such plants (Table 6.1) the C3 plants should be the least adapted to hot desert environments. Table 6.1 Comparison of the General Properties of C3, C^, and CAM Plants C3

C4

CAM

(1) Light saturation

1/4 full sunlight

Fixes C at night

(2) Optimum temperature (3) Water loss rate (gHÔg-iC fixed) (4) Photosynthetic rate (mgC02dm-2leaf-^h-i)

Around 25°C

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Around 600

Around 250

Around 50

25

60

3

Around 35°C

Adaptations C3 photosynthesis:

135

CO2 + RuBP -^ 2 PGA (3C) Carbon dioxide plus a five-carbon molecule, ribulose biphosphate, yields two molecules of a three-carbon molecule, phosphoglyceraldehyde.

C4 photosynthesis:

CO2 + PEP (3C) -^ OAA (4C) or M (4C) Carbon dioxide plus a three-carbon molecule, phosphoenolpyruvate yields one of two four-carbon molecules, oxaloacetic acid or malate. OAA —>- malate

t

1

CAM photosynthesis: CO2 C4 cycle i ^ CO2 + RuBP -^ PGA -^ glucose

PEP - 3 m) plus numerous fine shallow roots emanating from the root crown. This rooting pattern allows this shrub to use soil water from small rain events and to access soil nutrients via the shallow roots. Most of the soil nutrients are concentrated in the upper 10 cm of soil which are therefore available to the shallow roots. The deeper roots act as channels for stemflow water. Canopy morphology affects the fraction of intercepted rainfall that moves along stems to the root crown and then down root channels. Stem angle and stem length accounted for 41% of the variabihty in measured stem flow (Whitford et al, 1997). Stemflow volumes were higher on stems with angles equal to or greater than 65° and little or no stemflow was collected from branches with stem angles equal to or less than 50°. Canopy morphology affects subcanopy soil characterisitics. Shrubs with inverted cone mophologies do not accumulate litter layers under the canopy whereas those with a hemispherical shape develop litter layers and elevated nitrogen in subcanopy soils (DeSoyza et ai, 1997). Examining the traits of Larrea tridentata that contribute to its success in hot deserts provides a suite of characteristics which may be available to other C3 shrubs and small trees inhabiting hot arid environments. Although it is unUkely that any other species will share all of these traits, it is quite probable that some subset will apply. It is also likely that there are traits of other species not shared with creosotebush that confer some advantage to those species in hot, arid environments. Perennial plants can be divided into functional groups based on shared morphological and physiological characteristics. Many arid adapted plants are succulents in which water is stored in stems or modified leaves (cladodes). Examples of succulents include the Cactaceae and many of the Euphorbiaceae. The primary advantage of succulence is that the stored water can be used during extended dry periods to keep the photosynthetic area alive. Many succulent plants utilize the CAM photosynthetic pathway. The CAM pathway reduces water loss per unit carbon gained. In CAM photosynthesis, the stomates open at night. The extent of stomatal opening is governed by ambient temperature, plant water status, internal carbon dioxide concentration, duration of prior light exposure, ambient humidity, and age of the photosynthetic tissue. The carbon that is accumulated as malate during the night is stored in vacuoles and recycled into the photosynthetic process during daylight hours. The high water content of cell vacuoles dilutes the concentration of malate which allows sufficient malate accumulation to support carbon fixation into glucose during the day. Most of the stem succulents tend to have shallow root systems and are capable of exploiting water resulting from small rain events. Rapid rehydration allows optimum use of soil water. Many have spines and surface hairs that reduce tissue temperatures and also protect the plants from herbivores.

138


Although the shallow roots of succulents can allow rapid uptake of water and replenishment of tissue water storage even after light rain, they can also be pathways for water loss to drying soil. However, this avenue of water loss seems to be minimal in many succulents. For example, studies of two succulents from the Sonoran Desert (Agave deserti and Ferocactus acanthodes) lost very little water through the roots during drought (Jordan and Nobel, 1984). These succulents exhibited root elongation within 12 days after a large rainfall. The rapid root growth was balanced by root death during subsequent dry periods. Roots of these succulents decreased in volume during drought and exhibited little water loss to the surrounding soil (Jordan and Nobel, 1984). Almost all plants with the CAM photosynthetic pathway are succulents but not all succulent plants use the CAM carbon fixation pathway. There are two major categories of succulent plants: leaf succulents and stem succulents. Leaf succulents occur in arid saline environments but are not restricted to saline soils. Many succulent Chenopodiaceae occupy saline soils. Examples of saline soil stem succulents include Haloxylon, Hammada, and Anabasis and saline soil leaf succulents of the genera Suaeda, Seidlitzia, and Traganum (Shmida, 1985). Ions are sequestered in cell vacuoles which allows processes within the cytoplasm to proceed without the interference of these ions. The ions stored in vacuoles of succulents also establish a higher water potential gradient between the soil and leaves. This may result in leaves becoming more succulent than they would under nonsaline conditions (Jennings, 1976). Cactus-like plants are generally considered to be arid-adapted plants. These plants generally have succulent stems with large amounts of storage parenchyma and leaves are short-lived or absent. Genera that are subdominants or dominants in desert areas include Opuntia and Carnegia in North America and Euphorbia in southern Africa (Fig. 6.5). Because succulents cannot lower their tissue temperature by evapotranspiration, their tissues must tolerate higher temperatures than other plants in the same environment (Nobel, 1984). The high tissue water potentials of CAM plants limit their capacity to extract water from soil below -0.10 to -0.15 Mpa. Most stem succulents have roots concentrated near the soil surface where they can rapidly absorb water from small rain events. Because of the high water content of their tissues, few stem succulents can withstand long periods of below freezing temperatures. Stem succulents also die if desiccated. Recently established juvenile stem succulents are particularly susceptible to freezing and extended periods of drought. These characteristics explain the distribution of stem succulent CAM plants (Fig. 6.6) Woody shrubs and small trees may use either C3 or C^ photosynthetic pathways. Many species are deep rooted; roots of some species have been reported at depths of >10 m. Deep-rooted shrubs frequently have some roots very near the surface of the soil. The separation of the functional roots into shallow and very deep roots is an essential feature of woody shrubs or trees that exhibit hydraulic lift. Hydraulic lift is a process in which water that is transported from deep in the soil to the root mass near the surface, diffuses into the soil at

Adaptations

Figure 6.5

139

A Euphorbia spp. succulent from northwestern South Africa.

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Figure 6.6

The geographic distribution of CAM succulents.

night when the thermal characteristics of the soil favour diffusion out of the root system. During the daylight hours, that water is absorbed by the shallow roots and IS used to support stomatal opening for a short period of time. This allows some photosynthesis to occur in plants when the upper 30 cm of the soil is too dry for plant roots to absorb moisture.

140


Deep-rooted shrubs tend to be phenologically predictable. For example mesquite (Prosopis glandulosa) produces new leaves during the late spring. Shortly after new foliage is added, the plants produce flowers and fruits. Although there is variation in the production of new stem leaders following wet and dry winter seasons, the timing of stem elongation and flowering remains relatively constant from year to year. Quantitative phenological measurements of mesquite in the Sonoran Desert of California showed that leaf production and stem elongation were rapid and of short duration (< 3 weeks). Mesquite is a C3 shrub and like creosotebush produces two cohorts of leaves. The second cohort of leaves is only a small proportion of the first cohort and the survivorship of second cohort leaves is significantly shorter than of first cohort leaves in the California population (Nilsen et ai, 1987). When mesquite was subjected to drought stress, its leaf water potential responded to a large rain event as rapidly as unstressed plants (Ansley et aL, 1992). This rapid response in drought-stressed plants demonstrates the importance of shallow roots in addition to the deep roots of a desert shrub. All of the Mojave Desert shrubs studied by Turner and Randall (1987) exhibited predictable phenology of leaf growth. Leaves were produced in all species of shrubs each year of the study despite large differences in the timing and amount of winter rainfall. Shrubs of the genera Ambrosia, Grayia, Krameria, Larrea, and Lycium initiated leaf production in February and March. However, flowering was considerably more temporally variable (March-May) and in some years shrubs that flowered did not produce fruits. Plasticity in flowering and fruiting is probably necessary in desert environments where seasonal rainfall may be too low to support sufficient carbon storage for flower and fruit production. An interesting and important characteristic of desert shrubs is the capacity to allow branches to die and retain some branches as live stems. Studies of water relations of shrubs in the Tunisian Desert of North Africa showed that there were marked differences in water uptake patterns in two shrubs (Ourcival et aL, 1994). One shrub, Artemisia herba-alba is a shallow rooted shrub with the top fine roots at 2.5 cm depth. The other shrub, Anthyllis henoniana is a deep-rooted legume with the top fine roots at 8 cm depth. Anthyllis takes up newly available water more rapidly than Artemisia despite having fewer surficial roots. The main stem of Artemisia is divided into physiologically independent branches. Any one or several of the branches may die during an extended hot, dry period without resulting in the death of the rest of the plant. This variable stem death has been observed in other shrub species: Artemisia campestris, Salsola vermiculata, and Rantherium sanveolens. Shrubs that exhibit this physiological characteristic are considered to transport water in sectorial winding ascent (Waisel et aL, 1972) where water transported by the roots is absorbed by a branch that is not necessarily directly above that root. The result of sectorial winding ascent of water is that different sectors of the canopy have different leaf water potentials at the same soil water potential. This allows one sector of the canopy to die during drought thereby avoiding the death of the whole plant.

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Desert perennial grasses exhibit very different responses to water availability and ambient temperatures. Although most desert grasses are C^, some respond to soil moisture at temperatures that are too low for the growth of most desert grasses. A South African grass, Eragrostis lehmanniana (Lehmann's love grass) has spread rapidly throughout southeastern Arizona and southern New Mexico (Anable et al, 1992). This species has been recorded producing flowering tillers and seeds in response to a large November rain event. None of the other grasses produced flowering tillers and only a few of the other grasses produced green leaves. At low ambient temperatures, most C^ grasses are dormant but there are some species such as Lehmann lovegrass that are exceptions to that generalization. The high temperature-high light intensity characteristics of C^ plants would suggest that species with this photosynthetic pathway or the CAM pathway would dominate the vegetation in hot deserts. However, most of the studies of photosynthetic pathways of the flora of desert regions show that plants with the C3 photosynthetic pathway dominate both numerically and in terms of cover and biomass (Syvertsen et ai, 1976; Zeigler et ai, 1981; Hattersley, 1983; Vogel et a/., 1986). Although the C^ pathway is more common among grasses than in shrubs or herbaceous plants, many of the desert grasses are C3. In winter rainfall areas, such as the Negev, Saudi Arabia, and the Namib, C3 species have an advantage because they can grow at minimum temperatures far below those required for growth of C4 species. Hot desert perennial C^ grasses exhibit differences in responses to extended drought. Some species apparently mobilize reserves from tillers in the center of the clump which results in the death of tillers from the center outward. After a drought, the surviving grasses are rings or partial rings of live tillers. This has been observed in dropseeds (Spowbolus spp.), three-awns (Aristida spp.), and fluff grass (Dasyochloa [Erioneuron] pulchella). In other species such as blackgrama, Bouteloua eriopoda, tiller death occurred at the periphery of the clump with the central tillers surviving the drought. In all of these species there is a positive relationship between size of grass clump and survivorship. 6.2.2 Reptiles Maintaining water and electrolyte balance is a problem that herbivorous reptiles have solved by physiological traits. Several species of desert lizards, Uromastix acanthinurus, the chuckwalla, Sauromalus obesus and the desert iguana, Dipsosaurus dorsalis, share physiological traits that allow them to use plant parts as a food source (Grenot, 1968; Vernet et a/., 1988). Plant tissues have high concentrations of potassium that must be excreted in order for the animals to maintain electrolyte balance. Herbivorous lizards have salt-excreting glands that drain into the nares. These glands secrete specific salts against a concentration gradient thereby serving as extrarenal kidneys. The salts eliminated from the salt glands of herbivorous reptiles are mostly potassium bicarbonates. Insectivorous/carnivorous lizards excrete sodium chloride from the nasal salt glands (Vernet et ai, 1988).

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Accumulating fat at the base of the tail is another adaptation of some herbivorous reptiles. Both Uromastix spp. from North Africa and Sauromalus obesus from North America accumulate large reserves of fat at the base of the tail during the brief period when flowers and fruits of ephemeral and perennial plants are available. These fat stores allow these species to survive extended drought periods when little or no suitable plant material is available. The North American desert tortoise, Gopherus agassizii, is a herbivorous reptile that has no functional nasal salt gland. This species tolerates water and salt imbalances on a daily basis. When green vegetation is available, desert tortoises consume large quantities, which results in high concentrations of potassium salts in their plasma. The tortoises tolerate this osmotic imbalance and when ephemeral vegetation dries out, they shift to perennial grasses, which have low water content. When rainwater is available, the tortoises empty their bladders, drink copiously and store water in their bladders (Nagy, 1988). The capacity to tolerate large shifts in osmotic concentrations in the plasma and to store large amounts of water in the bladder allow this species to maintain an herbivorous life style in an extremly arid environment. 6.2.3 Invertebrates Although there are few morpological or physiological specializations of invertebrates that are clearly different in desert dwelling species in comparison to related species inhabiting mesic environments, there are some that have been studied and which deserve mention. Tenebrionid or darkling beetles are numerous and conspicuous members of the invertebrate faunas of deserts throughout the world. For example, the Namib in southwest Africa is reported to support 90 genera (Koch, 1962). Desert tenebrionids are flightless with fused elytra enclosing an air space called the subelytral cavity. Most mesic species have functional wings and moveable elytra. Studies by Cloudsley-Thompson (1964) and Ahearn and Hadley (1969) demonstrated that the subelytral cavity was an important mophological adaptation that reduced water loss by evaporation from the integument. They hypothesized that evaporation was decreased because the abdominal spiracles open into the relatively humid subelytral cavity rather than to the outside air. By opening into the subelytral cavity, the spiracles are protected from the drying effects of wind. The subelytral cavity also reduces the vapor pressure gradient between the trachae and their external environment thereby reducing evaporative water loss via the spiracles. An alternative hypothesis was offered by Fiori (1977) and Slobodchikoff and Wisman (1981) who suggested that the subelytral cavity allows abdominal expansion which may be important for species that feed on detritus. The low nutritional value of dead plant material (detritus) requires the animals to ingest large volumes of material to meet energy needs. Thus an expandable volume digestive system would be adaptive. An alternative advantage of an expandable abdomen is the capability to store large quantities of water in the abdominal cavity. All of these characteristics of the subelytral cavity are adaptive

Adaptations

143

for detritus feeding tenebrionid beetles that inhabit hot arid environments (Draney, 1993). Tenebrionid beetles in the Namib generate a wax bloom (wax extruded from the pores in the elytra that forms a powdery or filamentous covering of the surface). Wax bloom is produced in response to high temperature and low humidity (McClain et ai, 1984). The wax bloom protects diurnal beetles against the high temperatures and radiant heat loads. Wax blooms may occur in tenebrionid beetles in other deserts but this has not been reported in the literature. Perhaps the most interesting adaptation to desert environments is cryptobiosis. Animals that enter a cryptobiotic state exhibit marked reduction in body water content and extremely low metabolic rates. When water becomes available, these animals rehydrate quickly and return to normal activity. Cloudsley-Thompson (1991) reports one example of the larvae of an insect that exhibits cryptobiosis. The larvae of a chrinomid midge, Polypedilum vanderplanki, inhabit shallow rock pools in the semiarid regions of Nigeria. The rock pools fill and dry several times during the rainy season. When the pools fill, the desiccated larvae absorb water through their cuticle and begin feeding within an hour. Some Australian chrinomid larvae may also tolerate complete dehydration and rehydrate with no apparent physiological damage (Cloudsley-Thompson, 1991). Cryptobiosis has been reported in soil mites (MacKay et al., 1986). Cryptobiosis in Collembola and in Nematoda is referred to as anhydrobiosis (literally life without water). Soil nematodes lose water rapidly in drying soil and change into a coiled, dehydrated form (anhydrobiotic). When anhydrobiotic nematodes are placed in water, they rehydrate in less than one hour and resume all of their activities including reproduction (Demeure et al, 1979). Collembola do not coil but do dehydrate and survive with immeasureable metabolism (PoinsotBalaguer, 1984). Cryptobiosis or anhydrobiosis is an important adaptation for desert soil mesofauna because it allows these organisms to reproduce and expand their populations in response to available food sources rather than be constrained by the short pulses of suitable environmental conditions to gain sufficient energy to produce eggs and reproduce. 6.2.4 Large Mammals Since large grazing animals like camels, oryx, sheep, etc., do not have the option of seeking moderate microclimates below ground, they are subjected to the full effect of the high radiative, thermally desiccating environment of the desert. These environmental effects can be summarized by examining the general energy budget equation for an animal:

M±K±R±C-E=S where M is heat production by metabolism, K is energy gained from or lost to the environment by conduction, R is energy gained or lost by radiation, C is energy gained or lost by convection, E is energy loss by evaporation, and S is heat storage.

144


m iyfiiiglit

Wind |«©rifieti§ii|

Conduction to or from soil

Figure 6.7 Diagram of the energy exchanges of a medium-size desert mammal.

When an organism is in thermal equilibrium, S, heat storage, will equal zero. These exchanges are summarized in Fig. 6.7. Of the energy fluxes, radiative heating is the largest heat gain during the day and an avenue for heat loss at night. Many large desert animals store fat in localized areas of the body which enhances radiative and convectional heat fluxes (e.g. the camel's hump and fat-tailed sheep). Fat storage in the hump, or tail provides an energy reserve without the insulation that would result from subcutaneous fat storage. An insulative subcutaneous fat would reduce the rates of radiative heat flux from the body surface. Some large desert mammals have labile body temperatures which allow the animals to store heat during the day without the necessity of expending water for evaporative cooling. These animals subsequently unload heat at night by radiation to the cool night sky and convectional cooling. In some species the body temperature is allowed to drop below 38°C, e.g. camels (Gauthier-Pilters and Dagg, 1981), eland and oryx (Fig. 6.8) (Taylor, 1969). In these animals the body temperature may fluctuate as much as 7-8°C, i.e. 33.9^2.1°C. Taylor (1969) reported that when dehydrated oryx were exposed to 45 °C the animals did not sweat and the body temperature exceeded 45°C for 8 h. A body temperature of 42°C is lethal for most mammals. These large desert herbivores apparently survive such

Adaptations

• * '

145

.^•".•'t'

^ ^ ^ 1 •

"

^

^

.

.

:

'

.

;

'Jf^r^'Â

J!,;:; 'm^^'^^, ,fe^'

Figure 6.8

An oryx, Oryx gazella, in the Kalahari Desert, South Africa.

high body temperatures by maintaining lower brain temperature by means of a countercurrent heat-exchange system at the base of the brain, which cools the blood going to the brain via the carotid artery. Taylor (1969) describes a carotid rete system that utilizes a countercurrent heat-exchange rete at the base of the brain to cool the blood going to the brain (Fig. 6.9). The venous return from the nasal sinuses is several degrees cooler than arterial blood from the heart. The countercurrent heat exchange within the rete cools the blood flowing to the brain allowing the brain to operate at sublethal temperatures while the main body mass is at a temperature that would kill brain tissue. In addition to a labile body temperature, and modified fat storage, large desert herbivores may have one or more additional physiological characteristics that reduce the frequency of drinking or the dependence upon drinking for the replacement of water. Consider the avenues of water gain and loss in animals (Table 6.2). Water may be gained preformed in food, as an end product of metabolism or by drinking (free water). Metabolic water production varies directly as a function of metabolic rate and hence is a meaningful source of water only for small homeotherms which have a suite of water-conserving behavioral and physiological adaptations, e.g. North American kangaroo rats. Large grazing herbivores must obtain water by drinking or preformed in food. In an environment where sources of free water are sparse and unreliable, replacement of water lost in the urine, feces and by evaporation can be accomplished largely by preformed water in the food.

146

Ecology of Desert Systems T Brain 40.6 ^C

Cooled venous blood Nasal sinuses (evaporative cooling)

TBody 43*»C

40

39

i 38

'' 3C^

41

42

i

''

r

40

41

42

43

Carotid blood flow Venous blood from nasal sinus

Figure 6.9 Diagram of the brain rete and the temperature changes in a countercurrent vascular system. (Modified from Taylor, 1969.)

Table 6.2 Water Balance in Mammals Listed in the Sequence of Relative Importance in Most Mammals Water gain Free water (drinking) Preformed water in food Metabolic water

Water loss Evaporation from skin surface Evaporation from respiratory surfaces Fecal water Excretory (urinary) water

However, preformed water in food depends on the availability of quality forage. Low quality forage (woody stems, stem cured grass) contains little water and has lower digestibility thereby increasing fecal water loss (Gauthier-Pilters and Dagg, 1981). Large herbivores must also be able to withstand marked water loss without explosive and lethal rise in body temperature. For example, camels can lose more than 25% of their body weight without serious physiological

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consequences and rapidly rehydrate when water becomes available. Camels also lose water slowly. The dense wool-like hair of the camel establishes a steep thermal gradient that reduces the skin surface temperature and also provides a steep humidity gradient. The steep temperature-moisture gradients across a dense fur (hair layer) reduces insensible evaporative water loss and reduces the rate of heat gain of an animal exposed to the intense thermal environment of a mid-day summer. Kangaroos have a ruminant-like digestion and specialized dentition that allow them to utilize high-fiber, low-nutritive plants. Water loss via urine, feces and evaporation is reduced by a relatively efficient kidney, a capacity to recycle urea, and an ability to maintain plasma volume. Because of higher urea reabsorption from the glomerularfiltrate,the euro is able to process lower quality food than the red kangaroo. The red kangaroo (Macropus rufus) moves over wide areas of the arid region, seeking fresh green shoots that result from scattered storms. The euros {Macropus robustus) are more sedentary since they can utilize the lower nitrogen content of dead plant materials. The large Australian marsupials have evolved a reproductive strategy that allows them to cope with unpredictable climate and extended droughts. Joeys are bom at about one month of age in kangaroos and attach to a teat in the marsupium. If the mother is unable to obtain sufficient nutrition, the young is aborted with little strain on the mother. Marsupials also undergo embryonic diapause in which a viable embryo is carried in utero in an arrested blastocyst stage. The embryo begins further development when stimulus from a suckling young disappears. In a drought, female marsupials may have a succession of young that die around 2 months of age and are replaced. Under severe conditions reproduction may cease completely (Denny, 1982). In some large herbivores, urinary water loss may decrease as environmental stress increases. Camels, Bedouin goats and kangaroos conserve nitrogen by recycling urea back into the fermentation structures of the gut (Schmidt-Nielsen, 1959; Dawson, 1977) (Fig. 6.10). This allows these herbivores to use high C:N ratio (low quality, low protein, mostly cellulose) forage and maintain protein balance. Since the urea is returned to the gut rather than excreted, urinary water loss is reduced. One important set of physiological characteristics of the black Bedouin goat is associated with the ability of that species to continue lactation on low water inputs and on low quality diets (Shkolnik et aL, 1972). The abiUty to continue lactation under water deprivation is in part due to the ability of this species to overhydrate in comparison to other species of goats and sheep. The suites of adaptations of large herbivores may include physiological and morphological characteristics not described in the preceding discussion. It is important to re-emphasize that a large herbivore need not possess all of the characteristics described above to be 'adapted' to desert conditions. Although the suites of characteristics differ, the most successful large herbivores in arid ecosystems share a number of key features (Table 6.3).

148

Ecology of Desert Systems Plant material , (cellulose, lignins protein)

Kidney

Glucose Urea (lignins, cellulosic compounds) Figure 6.10 The resorption pathways of urea in herbivorous animals. (Modified from Schmidt-Nielsen, 1959.)

Table 6.3 Suites of Physiological and Morphological Characteristics of Three Large Desert Herbivores Camel

Bedouin goat

Red kangaroo

Yes > 20% of body mass Yes Yes

7 Yes Yes Yes

Yes Yes Yes Yes

Yes No Yes

No No Yes

No No Yes

Characteristic Labile body temperature Tolerate dehydration Thick wool-like hair Resorption of urea into digestive fermentation structures Specialized fat storage structure Sweating under thermal stress Rapid rehydration with infrequent drinking

6.2.5 Small Mammals Small, 8-150 g, seed-eating rodents are found in all subtropical deserts except in extremely arid deserts. These seed-eating rodents share a number of morphological and physiological traits that coupled with the behavioral traits discussed in the previous section provide a suite of adaptations that allow granivorous rodents to be successful inhabitants of deserts. The adaptation most important for granivorous rodents is the ability to maintain water balance and survive on a diet of seeds during periods when green plants and free water are not available. In order for a mammal to maintain water balance inputs by drinking, preformed water in food, and metabolic water production must equal or exceed evaporative water loss (primarily via respiratory surfaces) fecal loss and urinary loss. For small granivorous

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rodents evaporative water is the most important loss route and metabolic water production the most important input (MacMillen and Hinds, 1983). Urinary water loss is reduced to minimal quantities in desert rodents because of the morphology and physiology of the kidney. Most but not all desert-adapted rodents have kidneys with elongate renal papillae. The ability to survive on a diet of seeds without access to preformed water is a trait shared by a large number of small desert rodents (Withers et al, 1979; Hinds and MacMillen, 1985). Such rodents species frequently share the trait of an elongated rostrum which produces cooling of exhaled air via countercurrent vascularization of the nasal mucosa with the concomitant saving in respiratory evaporative water loss. Not all desert rodents are granivorous, nor do the granivorous rodents restrict their diets to seeds. It has been suggested that reproductive success in North American desert rodents is tied to the availability of green vegetation implying that the preformed water in such vegetation fills a physiological requirement associated with reproduction that cannot be met by metabolic water alone. This added water need is probably that water required for lactation. Other authors have suggested that chemical compounds in green vegetation stimulate reproductive activity in kangaroo rats (Kerley and Whitford, 1994). The importance of green vegetation for lactation rather than as a 'stimulant to reproduction' is supported by data on kangaroo rats. The testes length of male kangaroo rats, Dipodomys ordii, varies with temperature and not rainfall and the presence of embryos in females varies with rainfall of the previous month (Kerley and Whitford, 1994). In a seasonal desert, onset of rainfall during the normal wet season should be a good 'prediction' of the availability of green vegetation during the period of lactation in rodents that have less than a 40-day gestation period. Non-granivorous rodents require some characteristics in addition to those described for granivores. The chisel-toothed kangaroo rat, Dipodomys microps climbs into shrubs and harvests leaves of the halophyte Atriplex confertifolia (Kenagy, 1973). The epidermal tissues of these leaves are high in salts (electrolytes) but specialized photosynthetic parenchyma is low in electrolytes and high in starch. The lower incisors of D. microps are flattened and chisel shaped and are used to shave off the epidermal tissues in order to eat the inner tissue. The teeth of other kangaroo rats are rounded. The gerbilline rodent, Psammomys obesus of the Sahara and Middle East feeds on succulent parts of plants of the Chenopodiaceae, including some Atriplex. However, P. obesus consumes whole leaves but produces a more concentrated urine than D. microps. This example shows how differences in some characteristics of a suite of adaptations allows species to successfully use similar habitats and food. D. microps has morphological/behavioral traits and P. obesus physiological traits that all these species need to utilize leaf tissues with high electrolyte content. There are sufficient physiological data for desert rodents to make comparisons of the metabolic rates of desert and nondesert rodents. Goyal et al. (1982) reviewed the available data and developed regression equations for metabohsm of

150


desert and nondesert rodents. For burrowing desert rodents the regression equation M = 3.334W"^^^^ with a correlation coefficient (V = 0.82) was derived from the data in comparison with M = 8.853W"^ "^^^ for nondesert rodents. The lower basal metabolic rate reduces pulmonary water loss, reduces gas exchange in burrow environments that are poor in oxygen and rich in carbon dioxide and reduces or eliminates heat storage in saturated atmospheres (Goyal et ai, 1982). In a study that examined metabolic relationships in nondesert and desert heteromyid (kangaroo rats, pocket mice) rodents, Hinds and MacMillen (1985) reported that metabolic rates of the nine xeric species was 26% below but essentially parallel to the metabolic rate, body mass relationship of other eutherian animals. Another difference in desert and nondesert mammals is their weight specific water loss. As with metabolism, desert mammals tend to have lower rates of water loss than nondesert animals. 6.2.6 Carnivores Carnivorous mammals obtain preformed water in their food. Metabolic water contributes only a small percentage of the daily water requirements of intermediate and large size carnivores. For example Golightly and Ohmart (1984) reported that metabolic water contributed 18% of total daily water requirements of kit foxes (mean body weight = 2 kg) and 10% of the total daily water requirements of coyotes (mean body weight = 1 0 kg). They reported on energy to water requirement ratio, i.e. amount of prey required to meet energy needs divided by amount of prey to meet water requirements. For kit foxes this energy to water requirement ratio was 0.65 in winter and 0.57 in summer and for coyotes it was 0.30. Thus both of these species must either supplement their diet with succulent plant material as coyotes do or consume more food than is necessary for energy in order to meet their water requirements. 6.2.7 Birds There are few unique physiological adaptations of desert birds despite the stress imposed on organisms that fly in hot environments (Dawson, 1984). Hyperthermia (elevated body temperature) results from the metabolic heat production when flying limits the duration of flights of desert birds (Hudson and Bernstein, 1981). White-necked ravens, Corvus cryptoleucus, either avoid flying during hot conditions or fly at higher altitudes where the air is cooler. Some birds may be able to withstand limited hyperthermia because of the anatomical arrangement of blood vessels into a countercurrent heat exchanger. This has been described as a rete mirable opthalmicum in which the venous blood is cooled by evaporation from the buccopharyngeal mucosa and from the cornea (Bernstein et ai, 1979). Both desert and nondesert birds pant to eliminate excess body heat. In some species, e.g. caprimulgids (nighthawks, poor wills, frog-mouths), panting is

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151

augmented by gular flutter (Dawson, 1984). Water turnover in desert birds appears lower than in non-desert birds of the same size but Dawson (1984) urges caution in accepting this generalization because of differences in the conditions of the studies and diets among the species studied. Excretory water loss in birds is generally lower than in mammals because birds produce uric acid and uric acid salts that are eliminated as semisolids. Some desert birds possess nasal salt-secreting glands that allow excretion of salts with relatively little loss of water. Desert birds reported to have nasal salt glands include: ostrich, sand partridge, roadrunner and the Australian dotterel (Dawson, 1984). Salt-secreting glands are used by nestling roadrunners to compensate for the loss of water in evaporative cooling (Ohmart, 1972). These examples, although not an exhaustive list, serve to emphasize the point that many desert organisms avoid desert conditions. They avoid desert conditions by being active, growing, and reproducing only during wet, relatively cool periods, living in burrows in the soil, having crepuscular or nocturnal activity patterns, and/or selecting moderate microclimates in plant canopies. These behaviors allow many species with no obvious physiological adaptations to the extremes of desert environments to be successful inhabitants of such environments. For other species, avoidance behaviors are coupled with suites of morphological and physiological characteristics that are adaptive in dry hot and/or dry cold environments. Large herbivores are able to survive in hot desert environments because they have the capacity to maintain brain temperatures when body temperatures exceed 40°C. This frees the large herbivores from the cost of thermoregulation by evaporation of water.

REFERENCES Aheam, G. A. and Hadley, N. R (1969). The effects of temperature and humidity on water loss in two desert tenebrionid beetles, Eleodes armata and Cryptoglossa verrucosa. Comp. Biochem. Physiol. 30,739-750. Anable, M. E., McClaran, M. P., and Ruyle, G. B. (1992). Spread of introduced Lehmann lovegrass (Eragrostis lehmanniana Nees.) in southern Arizona, USA. Biol. Conserv. 61, 181-188. Ansley, R. J., Jacoby, R W., Meadors, C. H., and Lawrence, B. K. (1992). Soil and leaf water relations of differentially moisture-stressed honey mesquite (Prosopis glandulosa Torr). J. Arid Environ. 22, 142-159. Bennett, A. F, Huey, R. B., Henry, J. A., Nagy, K. A. (1984). The parasol tail and thermoregulatory behavior of the Cape Ground Squirrel Xerus inauris. Physiol. Zool. 57, 57-62. Bernstein, M. H., Curtis, M. B., and Hudson, D. M. (1979). Independence of brain and body temperatures in flying American kestrels, Ealco sparverius. Am. J. Physiol. 237, R58-R62. Burk, J. H. (1982). Phenology, germination and survival of desert ephemerals in Deep Canyon, Riverside County, California. Madrono 29,154-163. Cloudsley-Thompson, J. L. (1964). On the function of the sub-elytral cavity in desert Tenebrionidae (Col.). Entomol. Monthly Mag. 60, 3-7. Cloudsley-Thompson, J. L. (1965). Desert Life. Pergamon Press, Oxford. Cloudsley-Thompson, J. L. (1975). Adaptations of Arthropoda to arid environments. Annu. Rev. Entomol. 20, 261-283.

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Cloudsley-Thompson, J. L. (1991). Ecophysiology of Desert Arthropods and Reptiles. Springer Verlag, Berlin. Costa, W. R., Nagy, K. A., and Shoemaker, V. H. (1976). Observations of the behavior of jackrabbits (Lepus californicus) in the Mojave Desert. / Mammal. SI, 399-402. Cunningham, G. L. and Strain, B. R. (1969). An ecological significance of seasonal leave variability in a desert shrub. Ecology 50, 400-408. Davies, S. J. J. F. (1984). Nomadism as a response to desert conditions in Australia. /. Arid Environ. 7, 183-195. Dawson, T. J. (1977) Kangaroos. Sci. Am. lil, 78-89. Dawson, W. R. (1984). Physiological studies of desert birds: present and future considerations. /. Arid Environ. 7, 133-155. Demeure, Y., Freckman, D. W., and Van Gundy, S. D. (1979). Anhydrobiotic coiling of nematodes in soil. J. Nematol. 11, 189-195. Denny, M. J. S. (1982). Adaptations of the red kangaroo and euro (Macropodidae) to aridity. In W. R. Barker and R J. M. Greenslade (eds). Evolution of the Flora and Fauna of Arid Australia, pp 179-184. Peacock Publications, Glen Osmond, South AustraUa DeSoyza, A. G., Whitford, W. G., Martinez-Meza, E., and Van Zee, J. W. (1997). Variation in creosotebush (Larrea tridentata) canopy morphology in relation to habitat, soil fertility and associated annual plant communities. Am. Midi. Nat. 137, 13-26. Dimmitt, M. A. and Ruibal, R. (1980). Environmental correlates of emergence in spadefoot toads (Scaphiopus). J. Herpetol. 14, 21-29. Draney, M. L. (1993). The subelytral cavity of desert tenebrionids. Florida Entomologist 76,539-549. Ettershank, G., Ettershank, J. A., and Whitford, W. G. (1980). Location of food sources by subterranean termites. Environ. Entomol. 9, 645-648. Fiori, G. (1977). La cavita sottoelitrale dei tenebrionidi apomorfi. Redia 60, 1-112. Fisher, F M., Zak, J. C , Cunningham, G. L., and Whitford, W. G. (1988). Water and nitrogen effects on growth and allocation patterns of creosotebush in the northern Chihuahuan Desert. /. Range Mgmt 41, 387-391. Gauthier-Pilters, H. and Dagg, A. I. (1988). The Camel: Its Evolution, Ecology, Behavior and Relationship to Man. The University of Chicago Press, Chicago. Golightly, R. T. Jr. and Ohmart, R. D. (1984). Water economy of two desert canids: coyote and kit fox. J. Mammol. 65, 51-58. Goyal, S. P., Ghosh, P. K., and Prakash, I. (1982). Energetic aspects of adaptation in the Indian desert gerbil Meriones hurrianae Jerdon. J. Arid Environ. 5, 69-75. Grenot, C. (1968). Sur 1'excretion nasale de sels chez le lezard sharien: Uromastix acanthinurus. C. R. Acad. Sci. Paris 266, 1871-1874. Hattersley, P. W. (1983). The distribution of C3 and C4 grasses in Australia in relation to climate. Oecologia 57, 113-128. Hawke, S. D. and Farley, R. D. (1973). Ecology and behavior of the desert burrowing cockroach Arenivaga sp. (Dictyoptera, Poloyphagidae). Oecologia 11, 263-279. Hinds, D. S. and MacMillen, R. E. (1985). Scaling of energy metaboHsm and evaporative water loss in heteromyid rodents. Physiol. Zool. 58, 282-298. Hudson, D. M. and Bernstein, M. H. (1981). Temperature regulation and heat balance in flying whitenecked ravens, Corvus cryptoleucus. J. Exp. Biol. 90, 267-282 Huey, R. B, Pianka, E. R., and Hoffman, J. A, (1977). Seasonal patterns of thermoregulatory behaviour and body temperature of diurnal Kalahari lizards. Ecology 58, 1066-1075. Jennings, D. H. (1976). The effects of sodium chloride on higher plants. Biol. Rev. 51, 453-486. Johnson, K. A. and Whitford, W. G. (1975). Foraging ecology and relative importance of subterranean termites in Chihuahuan Desert ecosystems. Environ. Entomol. 4, 66-70. Jordan, P. W and Nobel, P. S. (1984). Thermal and water relations of roots of desert succulents. Ann. Bot. 54,105-111. Kay, R R. and Whitford, W G. (1978). Burrow environment of the banner-tailed kangaroo rat, Dipodomys spectabilis, in south-central New Mexico. Am. Midi. Nat. 99, 270-279

Adaptations

153

Kemp, P. R. (1989). Seed banks and vegetation processes in deserts. In M. A. Leek, V. T. Parker and R. L. Simpson (eds). Ecology of Soil Seed Banks, pp. 257-281. Academic Press, San Diego. Kenagy, G. J. (1973). Adaptations for leaf eating in the Great Basin Kangaroo Rat, Dipodomys microps. Oecologia 12, 383-412. Kerley, G. I. H. and Whitford, W. G. (1994). Desert-dwelling small mammals as granivores: intercontinental variations. Aust. J. Zool 42, 543-555. Killingbeck KT (1992) Inefficient nitrogen resorption in a population of ocotillo {Foquieria splendens), 3. drought-deciduous shrub. Southwest Nat. 37, 35-42. Killingbeck, K. T. (1993). Nutrient resorption in desert shrubs. Rev. Chilena Hist. Nat. 66, 345-355. Koch, C. (1962). The Tenebrionidae of southern Africa XXXI. Comprehensive notes on the tenebrionid fauna of the Namib Desert. Ann. Transvaal Museum 24, 61-103. Lajtha, K. and Whitford, W. G. (1989). The effect of water and nitrogen amendments on photosynthesis, leaf demography, and resource use efficiency in Larrea tridentata, a desert evergreen shrub. Oecologia 80, 341-348. Lee, A. K. and Mercer, E. H. (1967). Cocoon surrounding desert-dwelling frogs. Science 157, 87-88. Loveridge, J. P. and Craye, G. (1979). Cocoon formation in two species of Southern African frogs. SthAfr.J.Sci.lS,n-2Q. MacKay, W. P , Silva, S., Lightfoot, D., Pagani, M. S., and Whitford, W. G. (1986). Effect of increased soil moisture and reduced soil temperature on a desert soil arthropod community. Am. Midi Nat. 116, 45-56. MacMillen, R. E. and Hinds, D. S. (1983). Water regulatory efficiency in heteromyid rodents: a model and its application. Ecology 64, 152-164. McClain, E., Praetorius, R. L., Hanarhan, S. A., and Seely, M. K. (1984). Dynamics of the wax bloom of a seasonal Namib Desert tenebrionid, Cauricara phalangium (Coleoptera:Adesmiini). Oec(9/o^/fl 63, 314-319. McClanahan, L. L., Shoemaker, V. H., and Ruibal, R., (1976). Structure and function of the cocoon of a ceratophryd frog. Physiol. Zool. 56, 430-435. Meltzer, K. H. and Whitford, W. G. (1976). Changes in O^ consumption, body water and lipid in burrowed desert juvenile anurans. Herpetologica 32, 23-25. Moorhead, D. L., Fisher, F. M., and Whitford, W. G. (1988). Cover of spring annuals on nitrogen rich kangaroo rat mounds in a Chihuahauan Desert grassland. Am. Midi. Nat. 120, AA3-AA1. Mulroy, T. W. and Rundel, P. W. (1977). Annual plants: adaptations to desert environments. Bioscience 27, 109-114. Nagy, K. A. (1988). Seasonal patterns of water and energy balance in desert vertebrates. /. Arid Environ. 14, 201-210. Neufeld, H. S., Meinzer, R C , Wisdom, C. S., Sharifi, M. R., Rundel, P W , Neufled, M. S., Goldring, Y., and Cunningham, G. L. (1988). Canopy architecture oi Larrea tridentata (D. C.) Cov., a desert shrub: foliage orientation and direct beam radiation interception. Oecologia 75, 54-60. Nilsen, E. T., Sharifi, M. R., and Rundel, P. W (1984). Comparative water relations of phreatophytes in the Sonoran Desert of Cahfomia. Ecology 65,161-11%. Nilsen, E. T., Sharifi, M. R., Virginia, R. A., and Rundel, P W (1987). Phenology of warm desert phreatophytes: seasonal growth and herbivory in Prosopis glandulosa var. torreyana (honey mesquite). /. Arid Environ. 13, 217-229. Nobel, P. S. (1984). Extreme temperatures and thermal tolerances for seedlings of desert succulents. Oecologia 62, 982-990. Ohmart, R. D. (1972). Physiological and ecological observations concerning the salt-secreting glands of the roadmnner. Comp. Biochem. Physiol. 43A, 311-316. Ourcival, J. M., Floret, C , Le Floc'h, E., and Pontanier, R. (1994). Water relations between two perennial species in the steppes of southern Tunisia. /. Arid Environ. 28, 333-350.

154


Phillippi, T. (1993). Bet-hedging germination of desert annuals: variation among populations and maternal effects in Lepidium lasiocarpum. Am. Nat. 142, 488-507. Poinsot-Balaguer, N. (1984). Comportment des microarthropodes du sol a climat mediterranean francais. Bull. Soc. Bot. Fr. 131, 307-318. Ruiball, R., Tevis, L., and Roig, V. (1969). The terrestrial ecology of the spadefoot Scaphiopus hammondi. Copeia 1969: 571-584. Schmidt-Nielsen, K. (1959). The physiology of the camel. Sci. Am. 201,140-151. Seymour, R. S. and Lee, A. K. (1974). Physiological adaptations of anuran amphibians to aridity: AustraUan prospects. Aust. Zool. 18, 53-65. Shaffer, D. T and Whitford, W. G. (1981). Behavioral responses of a predator, the round-tailed Hzard, Phrynosoma modestum and its prey, honey pot ants, Myrmecocystus spp. Am. Midi Nat. 105, 209-216. Shkolnik, A., Borut, A., and Choshniak, I. (1972). Water economy of the Bedouin goat. Symp. Zool. Soc. Lond. 31, 229-242. Shmida, A. (1985). Biogeography of the desert floras of the world. In M. Evenari, I. Noy-Meir and D. W. Goodall (eds). Ecosystems of the World. Volume 12. Hot Deserts and Arid Shrublands. Elsevier, Amsterdam. Shoemaker, V. H. (1988). Physiological ecology of amphibians in arid environments. J. Arid Environ. 14,145-153. Slobodchikoff, C. N. and Wisman, K. (1981). A function of the subelytral chamber of tenebrionid beetles. / Exp. Biol. 90, 109-114. Smith, W. E. and Whitford, W. G. (1976). Seasonal activity and water loss relationships in four species of Eleodes (Coleoptera: Tenebrionidae). Southwest Entomol. 1, 161-163. Syvertsen, J. P., Nickell, G. L., Spellenberg, R. W., and Cunningham G. L. (1976). Carbon reduction pathways and standing crop in three Chihuahuan Desert plant communities. Southwest. Nat. 21, 311-320. Taylor, C. R. (1969). The eland and the oryx. Sci. Am. Ill, 89-96. Tevis, L. Jr (1958). A population of desert ephemerals germinated by less than one inch of rain. Ecology 36, 668-695. Tevis, L. Jr and Newell, I. M. (1962). Studies on the biology and seasonal cycle of the giant red velvet mite, Dinothrombium pandorae (Acari, Thrombidiidae). Ecology 43, 497-505. Turner, F. B. and Randall, D. C. (1987). The phenology of desert shrubs in southern Nevada. /. Arid Environ. 13, 119-128. Vemet, R., Lemire, M., Grenot, C. J., and Francaz, J-M (1988). Ecophysiological comparisons between two large Saharan lizards, Uromastix acanthinurus (Agamidae) and Varanus griseus (Varanidae). / Arid Environ. 14,187-200. Vogel, J. C , Fuls, A., and Danin, A. (1986). Geographical and environmental distribution of C3 and C4 grasses in the Sinai, Negev, and Judean Deserts. Oecologia 70, 258-265. Waisel, Y, Lipshitz, N., and Kuller, Z. (1972). Patterns of water movements in trees and shrubs. Ecology 53, 520-523. Westoby, M. (1981) How diversified seed germination behavior is selected. Am. Nat. 118, 882-885. Whitford, W G. and Ettershank, G. (1975). Factors affecting foraging activity in Chihuahuan Desert harvester ants. Environ. Entomol. 4, 689-696. Whitford, W G., Martinez-Turanzas, G., and Martinez-Meza, E. (1995). Persistence of desertified ecosystems: explanations and implications. Environ. Monit. Assess. 37, 319-332. Whitford, W. G., Anderson, J., and Rice, P. M. (1997). Stemflow contribution to the 'fertile island' effect in creosotebush, Larrea tridentata. J. Arid Environ. 35, 451-457. Withers, P. C , Lee, A. K., and Martin, R. W. (1979). Metabolism, respiration and evaporative water loss in the Australian hopping mouse Notomys alexis (Rodentia: Muridae). Aust. J. Zool. 27, 195-204. Wooten, R. C. Jr, Crawford, C. S., and Riddle, W. A. (1975). Behavioural thermoregulation of Orthoporus ornatus (Diplopoda: Spirostrepidae) in three desert habitats. Zool. J. Linn. Soc. 51, 59-74.

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Ziegler, H., Batanouny, K. H., Sankhla, N., Vyas, O. P., and Stichler, W. (1981). The photosynthetic pathway types of some desert plants from India, Saudi Arabia, Egypt, and Iraq. Oecologia 48, 93-99. Zwiefel, R. C. (1968). Reproductive biology of anurans of the arid southwest, with emphasis on adaptation of embryos to temperatures. Bull Am. Mus. Nat. Hist. 140,1-64.

Chapter 7

Primary Production

Primary production is the amount of solar energy converted into biomass via photosynthesis. That quantity of energy minus respiration and losses due to consumption by herbivores is net primary production. Net primary production is generally measured as above ground dry mass (biomass) at the end of the growing season. However, net primary production includes all leaf and stem production, root production, and increases in stem diameters of perennial dicots. Most of the literature values for net primary production are based on a subset of the actual total net primary production. Plant production in an ecosystem determines the energy available to consumers and to decomposers. That part of the primary production that can be harvested directly or indirectly by herbivores is usually limited to the aboveground parts of a few species. In arid ecosystems, the biomass production estimates are generally made for those species of plants that provide forage for domestic livestock. Thus the above-ground net primary production that serves as forage for livestock is the production estimate that drives management decisions. Those variables that affect net primary production must be identified and their contribution to the variation in primary production understood to provide a basis for developing sustainable use of arid and semiarid rangelands. If primary production varied according to the pulse-reserve paradigm, there would be a linear relationship between annual net above-ground production and growing season precipitation. Although there is no doubt that precipitation is the most important variable affecting primary production in deserts, it is not the only variable that affects production. The inability to predict annual above-ground net primary production (AAGNPP) and the lack of understanding of all of the variables that affect AAGNPP are major contributors to the failure of management systems to be sustainable. Understanding the complex interrelationships of environmental factors affecting net primary production and production of harvestable biomass is necessary if we are to avoid over-harvesting that leads to desertification. 7.1 MEASUREMENT OF NET PRIMARY PRODUCTION Given the need for accurate measurement of AAGNPP and the considerable effort that has been expended to obtain such measurement, the absence of standard.

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accurate methods is surprising. The apparent simpHcity of obtaining such data is deceptive. There are no universally accepted methods for measuring AAGNPP in any ecosystem. In deserts where the plant communities are composed of perennial dicots and moncots, and ephemeral herbs and grasses the problem of measuring AAGNPP is exacerbated. The variation in composition percentages of these different life forms affects the temporal patterns of production. Temporal and spatial variability of resource distribution across the landscape also affects patterns of production. Temporal and spatial variability change a superficially simple problem into a complex problem. Ludwig (1986) reviewed many of the inherent problems in making accurate measurements of net primary production in deserts. The method least prone to large errors is the harvest method. Reasonable estimates of AAGNPP can be obtained by harvesting plants within a series of quadrats when the plants are at the peak biomass stage. In plant communities with species composition of various life forms, differences in growth rates and timing of maturation can lead to serious underestimates of biomass production. When harvesting perennial grasses, it is necessary to separate previous year's production from the current year's production. For perennial shrubs or trees, there may be different seasonal patterns of leaf senescence and new leaf production. These are important variables that must be considered when designing a harvest method for estimating AAGNPP in a desert ecosystem. Biomass production estimates of perennial dicots are usually made by harvesting new leaders (terminal elongation of stems). However, harvesting new stems does not account for increased girth of stems. Conservative estimates of leaf production can be made by placing litter traps below the canopies of trees and shrubs and collecting the contents on a monthly basis. This method provides no estimates of stem production. A plotless harvest technique has some advantages over fixed-plot techniques in that much larger areas can be sampled in short periods of time. In the plotless techniques, densities of plant species are estimated by measuring distances to the nearest individual plant from a point in each of four quarters around the point (Cottam and Curtis, 1956). By harvesting the plant in each quarter, the biomass produced can be estimated from the density data and the mass of the harvested plants. Average mass of species a = number of individuals of species a m~^ x average mass of the harvested individual of species a AGNPP = Z mean biomass of species

a-n

As with harvested plots, this method is most appropriate for grasses and herbaceous plants but is less accurate for trees and shrubs. Accuracy of AAGNPP measurements can be improved by making a series of sequential harvests (Ludwig, 1986). Multiple harvests during a growing season can account for short pulses of growth and death that are missed by harvests at the end of the growing season. However, the additional costs of conducting sequential harvests frequently precludes this as an option.

Primary Production

159

Several groups of ecologists working in arid ecosystems have used nondestructive dimension analyses to estimate AAGNPP. Such measurements have the advantage of allowing sampling of the same quadrats and the same individual perennials over time. This is particularly important in experiments where relatively small plots are used to examine the long-term effects of variables such as rainfall and nutrients (irrigation and fertilization). For dimension analysis, a series of plants of varying sizes of each species are harvested after canopy size (usually 2 diameters) and height has been measured. The relationship between one or more of the dimension variables and dry mass is determined by regression analysis (Table 7.1). Repeated measurement of the dimensions of plants can be converted into biomass estimates per unit area from the regressions. In order to estimate the growth increments for trees and shrubs, it is necessary to obtain estimates of length of new stems, leaf mass per unit new stem, leaf mass per unit length of old stem, and numbers of elongating stems per tree or shrub. Typically, nodes are marked and growth measured from that reference mark. However, there are problems of flaring errors (Ludwig, 1986) when dimension analysis is used for estimating AGNPP of perennial dicots. Flaring errors occur because of the number of steps involved in estimating the number of stems with terminal leaders, number of leaders per stem, number of leaves, flowers, or fruits per leader, and biomass-length relationships of the stem leaders, making the biomass estimates and the error inherent in the estimates made at each step. There are errors associated with the plant density estimates, with the canopy size and volume estimates and with the estimates of numbers of growth points per plant. The estimate of number of growth points per unit volume or canopy area of plant has a large variance. When the error terms are combined, the error of the estimate may be close to 100% (Ludwig, 1986). Table 7.1 Examples of Best-fit Regression Equations Used to Estimate Biomass Production of Annual Plants. (Regressions from Ludwig et al. 1975 and Gutierrez and Whitford, 1987) Species (life form) Astragalus tephrodes (forb) Bouteloua aristidoides (grass) Euphorbia micromera (forb) Iva ambrosiaefolia (forb) Phacelia coerulia (forb) Larrea tridentata (evergreen shrub) Flourensia cernua (deciduous shrub) Ephedra trifurca (evergreen shrub) Gutierrezia sarothrae (evergreen subshrub) Zinnia acerosa (deciduous subshrub)

Equation

r^

B = 0.0062 + 0.0078 V = 0.045 + 0.016 V = 0.012+ 0.0033 A = 0.059 + 0.0022 V = 0.12 +0.006 y

0.99 0.97 0.87 0.91 0.98 0.98 0.95 0.99 0.91 0.84

= 1504A2

= = = =

1425 A 3249 y-481 V^ 460y 845 A

B, biomass in grams; A, canopy area in cm^ for forbs and grasses; in m^ for shrubs from the mean diameters. V, canopy volumes calculated from height and canopy area using volume of a spheroid or inverted cone depending on the morphology of the plant species.

160


7.2 COMPARISONS OF PRODUCTION ESTIMATES WITH MESIC ECOSYSTEMS It is easy to pass off the productivity of desert ecosystems as insignificant when compared to the productivity of more mesic ecosystems. Based on the hmited data used in global comparisons of primary production, it is evident that deserts are relatively unproductive places (Evenari et ai, 1976). The production figures quoted in most texts dealing with productivity are usually in the range of 0-250 g m~^ yr~^ This range of values do indeed seem insignificant when compared to the 600-1500 g m~^ yr~^ for temperate forests and the 1000-3500 g m~^ yr"^ for tropical rain forests. In addition the above-ground standing crop biomass is generally low in comparison to mesic terrestrial ecosystems. Since deserts are characterized by short and frequently unreHable growing seasons because of variabiUty in precipitation, cropping is not an option for obtaining food and fiber from the land. This largely reduces the harvestable food and fiber option to secondary production by domestic livestock. Harvestable production is therefore only a small fraction of the 0-250 g m~^ yr"^ and large land areas are required to sustain even low rates of offtake. Although net primary production is low in areas where rainfall is less than 100 mm yr~\ in many semiarid ecosystems, above-ground net production may approach that of temperate forests. For example, on a single watershed, Ludwig (1986) measured AAGNPP between 30 g m"^ and 592 g m~^ for a year in which the annual precipitation was 182 mm and the growing season rainfall a low 81 mm. On this desert watershed with its several vegetation communities distributed over the slope positions on the watershed, the AAGNPP varied from a low value within the range generally reported for scrub deserts to a high value that falls within the low end of the range for temperate forests. This example provides a cautionary note when generalizing about productivity of deserts. Although many landscape units in a desert may be relatively unproductive, there are often extremely productive units within the landscape. Productivity studies should include the entire spectrum of landscape units within the region of interest. Productivity estimates should be partitioned proportionately among the landscape units in order to obtain reliable estimates of AAGNPP. The water-use efficiency (annual net primary production divided by annual evapotranspiration) of desert vegetation (0.1-0.3 g AAGNPP/1000 g water) is considerably lower than forests (0.9-1.8 g AAGNPP/1000 g water), and shortgrass prairie (0.2-0.7 g AAGNPP/1000 g water) (Webb et al, 1978). For deserts, water use during the previous year was significantly correlated with the current year AAGNPP. The best correlation between water use and productivity was obtained by summing water use for both current and previous years (Webb et ai, 1978). The water-use efficiency relationship results from the higher proportion of ineffective rain events in arid regions, shrub species that utilize deep soil water, and rainfall redistribution processes. In Australian mulga (Acacia aneura) banded landscapes, production of grasses and herbaceous plants in the groves averaged 180 g m~^ in response to 630 mm of

Primary Production

161

rain two months prior to initiating measurements (Friedel, 1984). The long-term average annual precipitation in this area is 263 mm. Mulga litter production was estimated at 36 g m~^. Summing litter production and herbage production provided an underestimate of AGNPP at these sites because it did not account for stem elongation and increases in girth of the mulga. Comparison of the interannual variability in above-ground net primary production using data from eleven of the Long-Term Ecological Research sites across North America, showed that the greatest interannual variabihty in AGNPP (based on 6-23 years of measurement) occurred in grasslands. Forests were the least variable (Knapp and Smith, 2001). The interannual variability in AGNPP was not related to variability in precipitation. Maximum variability in AGNPP occurred in biomes where high potential growth rates of herbaceous vegetation were combined with moderate variability in precipitation. In this analysis, the desert site data were from a mixed shrub-grass site and the data for Chihuahuan Desert grassland was included in the analysis of variability in AGNPP as a grassland site. The interannual variability in AGNPP in Chihuahuan Desert shrublands should be lower than the variability in Chihuahuan Desert grasslands because the dominant C3 shrubs and the C3 ephemeral plants can utilize soil water from rainfalls during the months when low night-time temperatures preclude growth of the C^ grasses. The mean AAGNPP reported for black-grama grassland (9 years of data) was 229.1 ± 21.3 g m~^ and for mixed desert grassland (10 years of data) was 184.5 ± 14.7 g m"^. These values for AAGNPP are well within the range considered average for desert ecosystems. 7.3 RAIN USE EFFICIENCY The effectiveness of precipitation varies considerably as was discussed in Chapter 2. For example, in the arid and semiarid zones of Tunisia, the proportion of rain events yielding less than 10 mm per month increased from 25% at the 500 mm average annual precipitation region to 50% at the 300 mm average annual precipitation region and to 60% in areas with average annual precipitation less than 200 mm. In extremely arid regions (< 100 mm average annual precipitation) that proportion is 80% (LeHouerou, 1984). However in the regions south of the Sahara, in the Sahel and the Sudan, this trend is less marked (LeHouerou, 1984). Rain use efficiency has been proposed as a unifying concept in arid land ecology (LeHouerou, 1984). Rain use efficiency is calculated on the basis of biomass production per unit of rainfall. LeHouerou (1984) indicated that the assumption that AET (actual evapotranspiration) is equal to precipitation in arid and semiarid regions is valid only at the catchment basin or geographic area scale. The failure of rainfall at the plot or site level to be an adequate predictor of plant production or (AET) is the result of small-scale run-off, run-on patterns that modify soil water distribution spatially and that have an effect on patterns of nutrient availability. The exception to this generalization is sites located on deep, sandy soils or

162


in extremely flat terrain. These landscape types are not widespread in arid and semiarid regions of the world. LeHouerou (1984) suggested that in areas of sandy soils with average annual precipitation greater than 250-350 mm, there is a decrease in rain-use efficiency due to nutrient limitations. He reported that adding high nitrogen and medium phosphorus fertilizers might increase rainfall use efficiency two to five times. Occasionally potassium, sulfur, magnesium and calcium fertihzation can produce an increase in net production. LeHouerou (1984) concluded that attempts at regional integration of production and rainfall have limited success because they fail to incorporate all of the variables that affect productivity, i.e. soil permeability, soil fertility, run-off amounts, vegetation structure, interception and distribution of rainfall in time and space. He also pointed out the need to consider the 'biological year' not just the calendar year for predicting productivity in seasonally arid areas. These variables are important considerations when interpreting results from productivity studies. 7.4 BELOW-GROUND PRODUCTIVITY There are few studies of below-ground productivity in deserts. Measuring live root biomass is fraught with many difficulties and measuring root growth and root turnover is exceedingly difficult. In many desert communities the root/shoot ratios may be very large (Rodin and Bazilevich, 1967). Even data on root/shoot ratios are limited and subject to large errors of estimation. Bell et al. (1979) measured seasonal changes in root, shoot, and reproductive structure biomass in eight species of Mojave Desert winter annuals. Roots accounted for 12-22% of the total biomass until late infloweringwhen root biomass declined in all species. In these winter annuals, more biomass was allocated to reproduction (16-50%) than to roots. In comparison with annuals, root: shoot ratios reported for desert perennials exhibit considerable variation among species (Bell et a/., 1979). Root: shoot ratios for desert shrubs range from 0.15 to 8.0. Using a technique based on the dilution of natural isotopes of carbon in structural carbon of root systems during the growing season, Caldwell and Camp (1974) were able to estimate turnover of the root systems in two North American Great Basin Desert shrub communities. In a Ceratoides lanata community they estimated root production of 186 g m"^ yr~^ and shoot production of 64 g m"^ yr~^ In a sagebrush, Atriplex confertifolia, community they estimated root production of 443 g m"^ yr~^ and shoot production of 154 g m"^ yr~^ Root turnover was three times greater than above-ground biomass production (Caldwell and Camp, 1974). Root turnover undoubtedly accounts for a large fraction of the carbon gain by desert perennial plants and is an important variable in carbon budgets. However, data are needed from many more desert areas before the contribution of root turnover in energy flow and nutrient cycling processes can be accurately assessed.

Primary Production

163

7.5 MODELS OF AGNPP Models for predicting the responses of AAGNPP of desert ecosystems to abiotic variables were different from those that were applicable to forests and mesic ecosystems (Webb et al., 1978). In mesic systems productivity varies with foliar standing crop and the most important abiotic variables are short-wave radiation and annual average temperature. For deserts and short grass prairie the important model variables were annual precipitation and annual potential evapotranspiration. The desert and short grass steppe model had the form: AAGNPP = A X FSC (Sin/z [B^^^ {PPT- PET) ti\) where FSC is foliar standing crop in g m~^, ti is the mean weekly temperature (°C), PPT is annual precipitation, PET is annual potential evapotranspiration. Sin/z is a hyperbolic sine function. A and B are empirical parameters: A is a coefficient from the Hnear relationship of foliar standing crop and AGNPP; B is the shift in AGNPP that is dependent upon water availability and temperature. Despite the inclusion of empirical parameters in the model, not all of the data from the International Biome Program measurements fit the model. There was reasonable correspondence between the model and actual estimated AAGNPP for a Sonoran Desert site, for two years of data from a Great Basin Desert site, and two of three years for a northern Chihuahuan Desert site. The AAGNPP of the Chihuahuan Desert site for the year that did not fit the model was approximately three times the predicted value. Obviously the failure of the model to predict AAGNPP for all years for which estimates were available suggests that variables in addition to those included in the model must be affecting primary production. Variables that may be important for predicting AAGNPP include: season of precipitation, precipitation event duration, intensity and storm depth; time lapse since last rainfall, and seasonal temperatures. The most important of these variables are distribution of storm depths and seasonal precipitation that together provide a measure of effective rainfall. Seasonal rainfall is especially important for predicting productivity of desert regions where growing season is determined by temperature as well as rainfall. For example in the northern Chihuahuan Desert, the growing season for the dominant C^ grasses and herbaceous plants is very different from the growing season for the C3 shrubs and ephemerals. Annual precipitation and potential evapotranspiration can completely mask important seasonal differences in distribution of storms of different depths. In the Chihuahuan Desert, the growing season for the C3 component begins in November and continues through April. The summer growing season includes the remainder of the year. During the summer growing season, storm depth is important because most of the rainfall in the Chihuahuan Desert is from localized and intense convectional storms. If the summer rainfall has a preponderance of storms that produce less than 6 mm per event, there may be little if any soil moisture to support the growth of C^ grasses and ephemerals. The model was much better at predicting AAGNPP for areas where most of the precipitation was from low

164


intensity relatively long duration frontal storms. Low intensity rainfall has high potential for infiltration and storage therefore quantity of rainfall is a reasonable measure of water available for plant growth. Considering the variables that affect AAGNPP in deserts, a model that is a slight modification of that described by Seely (1978) provides a more accurate prediction. This model is a modification of the Webb et al. (1983) model. It incorporates a run-off, run-on term, which is obviously necessary to account for the differences in productivity resulting from landscape position. That model has the general form: AAGNPP = B([P±a]-

PET)™^^(K x FSC)

where 5 is a water use efficiency constant, P is precipitation in mm, a is water added or subtracted from P due to run-off or run-on, Ti is average ambient temperature, ^ is a constant that modifies FSC. FSC is the average foliar standing crop of perennials at the peak of the growing season. The average foliar standing crop is a scalar reflecting the photosynthetic potential of the vegetation in an ecosystem. For example, dense shrubs lining a wash (ephemeral stream channel) have a higher photosynthetic potential per unit area than scattered shrubs on an upland surface or scattered clumps of bunch grasses on a basin grassland. Run-off, run-on decrease or increase effective moisture therefore are added or subtracted from precipitation depending upon the landscape position as a donor or recipient patch. A number of soil parameters affect the value of a, i.e. slope, soil bulk density, vegetal cover, landscape position and the intensity-duration characteristics of precipitation (P). Another variable not accounted for in the model for AAGNPP is the time lag in nutrient availability in relation to rainfall events. Rainfall seasonality and recent history of rainfall events affects nutrient availabihty. Post-drought 'flush' of primary productivity is well known to desert pastoralists in North America and Australia. The large pulse of high quality plant growth following a drought breaking rain is an important part of the folk knowledge of desert pastoralists. This flush of growth is attributable to higher concentrations of available nitrogen and other nutrients in soils as a result of the low productivity during the drought. Regression models provided good predictions of net above-ground production by winter ephemerals in winter-rainfall Mojave Desert (Turner and Randall, 1989). They found that net above-ground production of winter annuals was best predicted by September to March rainfall (r^ = 0.83) but not with calendar year precipitation. The regression for calendar year precipitation accounted for only 12% of the variation in above ground production. They found that a nonlinear model provided the best fit for the net above-ground production of winter annuals using the September-March rainfall. log (AAGNP) = 1.98 log (pptSM) - 26.2) - 2.75 where AAGNP is above-ground net production of annuals in g m~^ and pptSM is September to March rainfall in mm.

Primary Production

165

A similar model explained 86% of AGNP for perennial shrubs using the rainfall of October (year n-l)to September. That model is: AAGNP = 0.30 (pptOS) - 6.12 where pptOS is the rainfall from October the previous year to September of the year in which measurements are made. Both models imply that some minimum amount of rainfall is required for new above-ground production during the spring growing season. For Mojave Desert annuals that minimum is 21 mm and for perennial shrubs, 37 mm. The concept of a minimum or threshold quantity of rainfall for the initiation of growth of desert vegetation is obviously important. The data on production by Mojave Desert shrubs is particularly instructive. Rainfall amounts above the threshold but less than the long-term annual average rainfall, resulted in no significant differences in production of shrubs (Fig. 7.1). Four species of Mojave Desert shrubs exhibited large increases in biomass production at annual rainfalls of around 200 mm and four of the species exhibited little increase in production from that recorded with rainfall just exceeding the threshold for growth (Fig. 7.1). The absence of a linear response in shrub production with rainfall shows that the responses of individual species cannot

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Rainfall (mm) Figure 7.1 Net primary production (NPP) of eight species of Mojave Desert shrubs, o, Larrea tridentata; A, Ambrosia dumosa; •, Atriplex confertifolia; A, Ephedra nevadensis; • , Grayia spinosa; • , Krameria parvifolia; + , Lycium andersonii; *, Lycium pallidum. (Data from Turner and Randall 1989.)

166


be generalized. This also suggests that primary production of desert vegetation does not respond as the pulse reserve model predicts. Several short-term studies concluded that primary production varies directly with rainfall. An Aristida pungens (perennial grsiSs)-Retama retam (shrub) community in the arid rangeland of Libya produced 200 g m~^ in a year with 200 mm rainfall and 95 g m~în a year with 171 mm of rainfall. Annual plants contributed 46 g m"^ and 30 g m~^, respectively, during those years (Gintzburger, 1986). Net primary production can also vary with age of the community. In the Chaco region in Argentina, natural vegetation communities recolonize abandoned dryland farms. The net primary production of an 8 year old 'chaco' woodland (2.0-11.5 g m~^) was higher than that of a 50 year old 'chaco' woodland dominated by Prosopisflexuosa(Braun-Wilke, 1982). 7.6 LANDSCAPE RELATIONSHIPS The best example of how patch location and precipitation variables affect AAGNPP in a desert is provided by Ludwig (1986) in a summary of five years of data from a variety of landscape units on a watershed in the northern Chihuahuan Desert. The data from the alluvial fan unit were the data used by Webb et al, (1983) in their modeling of AAGNPP for desert ecosystems. Ludwig's summarization shows that some of the patterns were attributable to seasonal patterns of rainfall and storm characteristics. His data showed that productivity on the uplands was greatly affected by the characteristics of the rain events of the growing season of a particular year. In a year when a large fraction of the growing season rainfall was a small number of high intensity, short duration storms, productivity on the upland slopes was considerably lower than the productivity in another year of lower annual rainfall but with storms of lower intensity and longer duration (Fig. 7.2). When the growing season rainfall was primarily high intensity, short duration storms, run-off from the upland units emptied into the drainage channels and initiated flow in the large drainage channels of the ephemeral streams (arroyos). The transmission loss water filled the sediments and provided a water source for high productivity of the channel margin vegetation (Fig. 7.2). Similarly the variability in AAGNPP on the basin slopes was not directly the result of amount of rainfall but largely due to the runon water from the upslope landscape units. This clearly demonstrates the necessity of including a run-off, run-on term in any model of productivity for desert ecosystems. An interesting comparison with the AAGNPP data shown in Fig. 7.2 can be made by examining data for a desert grassland located 10 km from the watershed studied by Ludwig (1986). The desert grassland is topographically flat with sandy soils. The soils have high infiltration characteristics and little if any potential for run-off or run-on. On this type of landscape unit, the timing and quantity of rainfall are the important determinants of productivity. The temporal pattern of rainfall determines which group of species responds.

Primary Production

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700 lake basin lower slope mid-slope large washes small washes alluvial fans

600 -I 500 H

> 400 -I E D) QQ-

300 200 100 H 0

150

200

250

300

350

400

450

Precipitation (mm)

Figure 7.2 The relationships between annual precipitation and annual above-ground net primary production (AAGNPP) on a variety of landscape units on a watershed in the northern Chihuahuan Desert. (Data from Ludwig 1986.)

Cool season rainfall results in no productivity by the C^ grasses that are the dominant perennials in this system. Cool season rainfall does stimulate production by winter-spring ephemerals (Table 7.2) Despite the absence of run-off or run-on, there was not a good correlation between annual rainfall or growing season rainfall and AAGNPP. The importance of run-off and run-on as a variable affecting primary production is not limited to North American Deserts. An analysis of AAGNPP for watersheds in the Negev Desert, Israel showed that production in Hammada communities at the base of the watershed was more a function of water gain by run-off floods than of actual precipitation (Evanari et aL, 1976). It had been shown that in dry years the soils of the Hammada communities received only 21% of the rainfall as run-on but in wet years this increased to 31% of the total precipitation. Here the importance of saturated soil increasing run-off is the explanation for the differences. In addition, much of the productivity in the Negev Desert communities in wet years was production by ephemeral plants. In relatively dry years there was little or no production of ephemeral plants. In the Hammada community, ephemeral plants accounted for nearly 64% of the biomass production in the wettest year of the study. In comparison, the production of ephemerals contributed an even larger percentage of the total biomass production in the Zygophyllum dumosum community on low slopes at near average rainfall for that area (Table 7.3).

168


Table 7.2 Annual and Seasonal Above-ground Net Primary Production (g m~^ yr~^) in a Desert Grassland for Three Consecutive Years. Annual rainfall and summer (growing season) rainfall are provided for water input comparisons. Data for shrubs primarily the subshrub Gutierrezia sarothrae. Winter-spring annuals and summer annuals are ephemeral herbaceous plants. (Data from Pieper et al, 1983.) AAGNPP (g m-2 yr-i) Year

1970 1971 1972

Annual rainfall (mm)

Summer June-Sept rainfall (mm)

Total

Shrubs

Winter-Spring annuals

Summer annuals

109 206 324

66.1 104 200

140 315 264

19 1 7

4 1 20

45.8 106.2 45.8

Table 7.3 Annual Above-ground Net Primary Production in Relationship to Rainfall in Three Plant Communities in the Negev Desert, Israel. Hammada scoparia communities are on the floodplains, Zygophyllum dumosum communities are on low sloping hills, and Artemesia herba-alba communities are on steep hillslopes. (Data from Evenari et al., 1976) Hammada scoparia

Artemesia herba-alba

Zygophyllum dumosum

Ann. rain^ (mm)

Biomass (gm-2)

Ann. rain (mm)

Biomass (gm-2)

Ann. rain (mm)

Biomass (gm-2)

94 163 54 134

192 (8) 420 (64) 39(0) 225 (51)

38 164

38 (0) 229(51)

52 93

38 (38) 211(71)

Ann. rain, annual rainfall in the year of the measurements; biomass, production. The numbers in parentheses are the percentage of the biomass production contributed by ephemeral plants.

Differences in AGNFF in different locations receiving the same annual rainfall were reported by Evenari et al. (1976). They concluded that the relationship between annual precipitation and primary production disappears in landscapes where topography and geomorphology affect infiltration, soil water storage, and run-off. The topographic position and geomorphological history of a landscape unit is an important determinant of the abundance and species composition of the subdominant plants in the community. They reported AGNFF of 250 kg ha~^ and 350 kg ha"^ for two Negev Desert communities with the shrub, Zygophyllum dumosum as a dominant and annual rainfall of 93 mm. The AGNFF of an Artemesia herba-alba community in the same region produced 440 kg ha~^ and a Hammada scoparia community in that region produced 1430 kg ha"^ in response

Primary Production

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to annual rainfall of 93 mm. A mixed A. herba-alba-Z. dumosum community had an AGNPP of 470 kg ha"^ with 93 mm of rainfall. The Z dumosum and A. herba-alba communities are on hillslopes and the H. scoparia community occupies a floodplain. An A. herba-alba community in Algeria produced an AGNPP of 177 kg ha~^ with 300 mm of annual rainfall and an A. herba-alba community in the northern Negev in Israel produced 360 kg ha~^ with 164 mm of rainfall (Evenari et a/., 1976). These studies clearly demonstrate the importance of landscape position and geomorphology-soil characteristics as variables affecting AGNPP when rainfall is equal at all of the sites. A long-term data set that provided information on productivity of severely degraded ecosystems in contrast with ecosystems that were in relatively good condition is that reported by Floret et al. (1983). Their studies were in a winter rainfall, scrub desert region of Tunisia. During the six years of their study, rainfall varied between 85 mm yr~^ and 431 mm yr~^ Their data showed a strong seasonality component to productivity, especially that of ephemeral plants (Fig. 7.3). They attributed much of the variation in AAGNPP to the seasonal distribution of rainfall. They also suggested that during wet years, nutrient availabihty, not water was the principal factor limiting productivity. On the most degraded site, all of the AAGNPP was contributed by ephemeral plants (Fig. 7.3). The fraction of AAGNPP contributed by ephemerals varied considerably among years and among sites (Fig. 7.4). Ephemeral plants accounted for all of the AAGNPP on the degraded erosion slopes.

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> wind and water erosion potential > loss of surface soil < infiltration > run-off Loss of part of soil seed bank < quality of environment for germination and seedling establishment

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The seed pods of many trees and shrubs that are numerically dominant in arid landscapes are consumed by native and domestic herbivorous vertebrates. The fruit pods of Prosopis flexuosa (Fabaceae) are consumed by domestic, exotic animals (horses and cattle), medium-size, herbivorous rodents (Lagostomus maximus Chinchillidae) and (Dolichotis patagonum (Caviidae)), and gray foxes {Pseudalopex griseus). Large numbers of P. flexuosa seeds were found in cattle and horse dung. Seeds that passed through cattle had lower viabihty but higher germination rates than seeds that passed through horses. Seeds passing through the herbivorous rodents had lower viability but higher germination rates than seeds collected from mature pods on the trees. Seeds passing through gray foxes did not differ in viability and germination from seeds collected from mature pods on the trees (Campos and Ojeda, 1997). Seeds often species of legumes and one species of grass survived passage through the digestive tract of cattle. Seeds of tussock grasses did not survive passage through cattle (Gardener et ah, 1993). Dispersal of problem shrubs such as mesquite {P. glandulosa, Fabaceae) into grassland habitats is one result of the introduction of domestic livestock into the desert rangelands of North America. Mesquite seedlings established from feces of domestic cattle, white-tailed deer {Odocoileus virginianus) and coyotes (Canis latrans) in semiarid grasslands and these consumers of mesquite fruits are considered vectors for the spread of mesquite into arid and semiarid grasslands (Kramp et ah, 1998) Other examples of animal dispersal of seeds of desert plants reviewed by Campos and Ojeda (1997) include dispersal of seeds of prickly pear cacti (Opuntia sp.) by jackrabbits (Lepus spp.) and by collared peccaries (Pecari tajacu). Some carnivores that exhibit seasonal consumption of fruits and seeds are effective dispersal agents. These carnivores include: badgers (Taxidea taxus) and coyotes {Canis latrans). Acacia fruits are consumed by a variety of animals that are probably effective dispersal agents for the seeds. In some desert plants, such as Opuntia sp. cacti, vertebrate consumption of fruits (frugivory) is the only mechanism of primary dispersal of seeds since all of the fruits are removed within a one-month period after ripening. In dense Opuntia dominated scrublands in Mexico, 100% of the fruits were consumed by birds and large mammals. The intensity of fruit removal was faster in areas with sparser cactus populations and in thedngst year of the study (Montiel and Montana, 2000). Mammals are not the only seed dispersers. Large birds that consume fruits can be effective dispersal agents. Australian emus (Dromarius novaehollandiae) are primary dispersal agents of nitre bush fruits. The salt-rich pericarps are removed as the fruits pass through the birds' digestive system. In addition the fruit pit is scarified which enhances germination (Noble, 1975).

8.6 GRANIVORY With the exception of the feeding behavior of domestic livestock, no other feeding behavior has attracted more research effort in arid and semiarid ecosystems than

Consumers, Consumption, and Secondary Production

207

seed feeding. Questions about the relative importance of seed-eating rodents, ants and birds have generated a large number of studies in North America and in other arid regions of the world (Kerley and Whitford, 1994). Interest in small manmials as granivores has been stimulated by the extensive research on North American heteromyid rodents, the best-studied desert small mammals. The diversity and abundance of heteromyid rodents, which are largely granivorous, has led to granivory being considered one of the key adaptations of small mammals to desert environments. A recent study of community structure in small mammals across four continents suggests that granivores are most abundant in some Northern Hemisphere temperate deserts (Kelt et al, 1996). The patterns of species composition included: low a diversity (two to four species per site), high (3 diversity (high species turnover between sites) and local coexistence of only 20-30% of the species in the regional pool. Deserts in the northern hemisphere had more granivores except for the Turkestan Desert where there were more foliage feeders. Omnivorous rodents dominated the small mammal communities in South America, Australia, and the Thar Desert of the Indian subcontinent (Kelt et aL, 1996). However, the demonstration that the process of granivory has major impacts on desert plant communities has focused attention on this process as critical in the functioning of desert ecosystems (Brown and Heske, 1990; Heske et aL, 1993; Guo et aL, 1995). Seed-harvesting ants, seed-feeding insects such as bruchid beetles, and birds also impact both the quantity and species composition of the seed reserves in the soil. The evaluation of the relative importance of various groups of seed feeders has been done by the use of seed bait experiments in which removal of seeds by different groups of seed feeders are measured (Brown et aL, 1975; Mares and Rosenzweig, 1978). The interpretation of the results of these experiments must be made with caution because the technique has some serious Hmitations (Parmenter et aL, 1984). The removal of seeds by ants and rodents does not reflect the timespecific seed requirements of these organisms because both taxa cache seeds for later consumption and caching is a common response to abundant seeds (Parmenter et aL, 1984). Birds require much longer time to locate dense patches of seeds because they rely on predictable, high quality food patches and spend little time locating new resources (Mares and Rosenzweig, 1978). Thus seed tray techniques measure the ability to locate and harvest seeds not granivory. The simplicity of the seed tray technique has allowed for a number of studies in arid regions. In North America and in Israel seed removal by rodents was considerably greater than seed removal by ants or birds (Fig. 8.7). However in a recent study, it was found that seed removal by ants was significantly higher than seed removal by rodents in Chihuahuan Desert grasslands but not in shrubdominated communities (Kerley and Whitford, 2000). In other desert regions seed removal by rodents was considerably lower than seed removal by harvester ants (Kerley and Whitford, 1994). The seed tray studies in North America were conducted in Chihuahuan Desert shrublands that had been desert grassland in the late 19th century. In a comparison of granivory in mesquite {Prosopis glandulosa) dunes, cresotebush {Larrea

208

Ecology of Desert Systems 10 1 T

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Figure 8.7 Comparison of seed removal from seed trays by rodents (•), ants (A), and birds (•) in desert areas for which there are pubhshed data. (From Kerley and Whitford, 1994.) Reprinted with permission of CSIRO Publishing.

tridentata) shrublands and desert grasslands using seed trays with large and small (< 1 mm diameter), grassland ants removed 14.2 g of seeds in 12 h. This rate of seed removal is the highest for any desert area in the world (Kerley and Whitford, 2000). Small native fire ants, Solenopsis xyloni, recruited rapidly to the seed trays and dominated the assemblage of ants visiting the trays. S. xyloni and Pheidole spp. removed large quantities of the small seeds and the larger seed harvester, Pogonomyrmex desertorum, removed larger seeds. Seed removal by ants was higher than by rodents in both grazed and ungrazed desert grassland sites. Seed removal by ants was lower at the shrubland sites than at the grassland sites. Rodents removed more seeds than ants in the creosotebush shrubland sites and slightly more than ants at the mesquite sites. The creosotebush sites had higher numbers of granivorous rodents than the mesquite and grassland sites. These patterns of seed removal by ants and rodents are a feature of vegetation changes resulting from desertification in the past 150 years (Kerley and Whitford, 2000). These results suggest that some of the differences in patterns of granivory in North American deserts in comparison with other desert areas of the world may be an artifact of the population responses to granivorous rodents to vegetation changes. In a study encompassing four years of monthly sampling of desert rodents, Whitford (1976) reported six species of seed-feeding heteromyid rodents and


209

eight species of omnivorous rodents (Fig. 8.8). One species, Onychomys torridus, is primarily carnivorous-insectivorous and two additional insectivorous species, the desert Notiosorex crawfordii and grasshopper mouse, Onychomys leucogaster, occur in the region but were not taken on the study areas. In a list of 16 species of rodents from the AustraHan arid zone, Morton and Baynes (1985), list five species that would be described as granivores but point out that in these species insects and plants make up part of the diet. This is also true of North American heteromyid rodents which consume variable quantities of insects and plant parts depending upon availability (Reichman, 1975). In that study, stomach contents were compared with cheek pouch contents. Cheek pouch contents were

(a)

SEED

HERBAGE

INSECTS

(C)

INSECTS

SEED

HERBAGE

INSECTS

SEED

SEED

HERBAGE

INSECTS

HERBAGE

Figure 8.8 Diets of small mammals for (a) North American desert (V, heteromyid rodents; •, nonheteromyids), (b) South American Atacama Desert, (c) Australian deserts, (d) South African Karoo. (From Kerley and Whitford 1994.) Reprinted with permission of CSIRO Publishing.

210


generally used to obtain dietary information for heteromyid rodents. The seedeating rodent species cited most frequently as a granivore (Dipodomys merriami) included insects as 15.5% of its diet and green vegetation made up 6.1% of its diet (Reichman, 1975). This is only one example of the variability in the diet of 'granivorous' rodents. Apparent 'plague' numbers of omnivorous rodents occur during particularly favorable conditions whereas the population densities of the more 'k' adapted heteromyids remain relatively constant. A conceptual model for this pattern of rodent abundance includes migration by omnivorous rodents into formerly unfavorable habitats and high, aseasonal reproduction by these species as factors accounting for the plague numbers of cricetid (murid) rodents following two wet years in succession (Whitford, 1976). Thus, although there are proportionately more species of 'granivorous' rodents in North American deserts, in the Chihuahuan Desert which has a climate most similar to the Alice Springs area, e.g. total rainfall, summer wet season and summer temperatures, the rodent assemblages of Australian deserts and the Chihuahuan Desert share many similarities. Comparative studies of granivory have been done using uncovered and screen mesh covered seed trays to measure relative rates of seed removal as an index of which group of granivores is most important in a given area (Brown et al, 1975; Mares and Rosenzweig, 1978; Morton, 1985; Kerley, 1991). In the Monte Desert of Argentina, seed removal rates were much higher from microsites under shrub canopies than in open, exposed microhabitats (De Casenave et a/., 1998). De Casenave et al. (1998) state that comparisons of seed removal rates among geographically disparate sites must be made with caution because the sampHng effort among microhabitats is frequently not reported. However, their study confirmed the conclusions of Mares and Rosenzweig (1978) that ants are the most important seed consumers in the Monte Desert and the total impact on seed reserves seems to be lower in South American and AustraHan deserts than in other desert areas of the world. In the Chilean matorral (semiarid shrubland) of South America, seed removal in sparse habitat was primarily by ants with little seed removal during the part of the year when ants were not active (Vasquez et al., 1995). In dense habitat seed removal by ants was equaled by seed removal by birds and mammals removed very little seed. Seed removal rates were correlated with abundance of ants in these habitats (Vasquez et al, 1995). Despite the differences in granivory in South American and Australian deserts, the seed reserves in these deserts are essentially equal to the seed reserves in North American deserts (Marone and Homo, 1997). Understanding granivory with respect to seed reserves will require rigorous studies on the impact of poorly studied granivores such as birds and on the effects of bacteria and fungi on seed reserves. The assessments of impacts of granivores on seed reserves will also require estimation of germination losses. Seed removal by mammals is consistently low in southern hemisphere sites. This pattern of seed consumption may be related to the paucity of granivorous rodents in southern continents (Kerley and Whitford, 1994) and/or to the more omnivorous food habits of South American small mammals (Kelt et al., 1996). In


211

South American and Australian deserts, ants were the dominant granivores. Marsh (1985) reported that most of the ants in the Namib were seed-harvesters and that the structure of the ant community was similar to that in Australia. Marsh's data suggest that ants are the most important seed consumers in the Namib. Kerley (1991) showed that in the Karoo (South Africa) ants removed considerably more seeds from trays than either birds or rodents. However, it is possible that the relative importance of ants and rodents as granivores has been misunderstood, especially in regions where rodent populations experience periodic explosive increases. Using a conservative energetics approach, it was estimated that seed consumption by rodents would be 30 times higher at a population episodic peak than was estimated by seed dish studies during periods of low rodent population densities. During peak populations of rodents in the Australian deserts, rodent consumption of seeds was estimated to be higher than the estimates for North American rodents and much higher than seed consumption by desert ants (Predavec, 1997). What is it about North American desert rodents that are different from rodents in other deserts? It is certainly not physiological because there are numerous reports of a wide variety of desert rodents having kidneys with long loops of Henle that allow maximal concentration of dissolved solids in the urine. Most if not all live in burrows where the microclimate is relatively humid and temperatures are moderate and most are nocturnal. One feature of heteromyid rodents that is different from most of the other groups of desert rodents are cheek pouches. Cheek pouch capacity is directly proportional to the body mass of the rodent. It has been estimated that a heteromyid rodent can transport a mass of seeds with the equivalent energy requirements of an individual for one day (Morton et ai, 1980; Vander Wall et aL, 1998). The cheek pouches allow heteromyid rodents to remain away from the burrow collecting seeds and fruits and storing them in the cheek pouches for later transport to the home burrow or to cache. The larger heteromyids collect seeds with all of the investing structures whereas the smaller species may husk the seeds and place only the edible parts in the cheek pouch (Leman, 1978). The overwhelming advantage of cheek pouches is the ability to collect and store quantities of seeds efficiently during periods of seed abundance. This allows the 'resident' individuals to survive long periods when there is little or no suitable seed production. Reichman (1975) suggested that heteromyids eat vegetation and insects as encountered but collect seeds for transport in cheek pouches at the same time. Thus cheek pouch contents do not necessarily reflect diet. External cheek pouches appear to be a feature of North American heteromyid rodents that is not shared by other rodent families. Seed-eating rodents that have no external cheek pouches must make numerous trips between a seed source and home burrow in order to build up a store of seeds or numerous trips to a cache site if seeds are to be cached. One study in Israel showed that the removal of seeds by rodents was nearly four times greater than seed removal by ants (Abramsky, 1983) (Fig. 8.7). The rodent community studied by Abramsky is dominated by gerbelline rodents

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(species with no external cheek pouches). These data suggest that external cheek pouches are not essential for rodents to specialize on seeds as a primary food source. Body size has an important effect on the diet composition of 'granivorous' rodents. Gerbilline rodents use green vegetation during the winter rainy season when seed reserves are lowest and green vegetation is available. Small gerbillines continue to consume seeds as a large fraction of their diet during winter whereas the large body size gerbilline {Meriones crassus) consumed green vegetation as the largest fraction of its diet (Degen et al, 1997). Seeds are the largest component of the diet of small gerbils and green vegetation is the largest component of the diet of a large gerbil. These differences have been attributed to the need for high quality, digestible food for small animals and the ability of larger animals to get by with lower quality food. They state that there is an inverse relationship between body mass and the ratio of seed:green vegetation intake in other Old World and New World granivorous rodents. In order to supply their metabolic needs, small desert rodents that have no cheek pouches must spend time foraging that is inversely related to the abundance of seeds or other food resources. When seeds, vegetation, and insects are sparse, small mammals are exposed to higher probability of being killed by predators. Predation has been suggested as one of the more important factors involved in structuring desert rodent communities (Price and Brown, 1983; Kotler, 1984; Bowers, 1990). Owls and snakes and several kinds of mammals are known to prey on desert rodents (Bouskila, 1995; Meserve et al., 1996; Gutierrez et aL, 1997). Predator exclusion studies have concluded that predation is the primary factor affecting the abundance of rodents and indirectly their long-term effects on vegetation (Meserve et al., 1993, 1996; Bock and Bock, 1994). Granivory during periods of depleted seed banks therefore exposes desert rodents that do not hoard seeds to greater risk of predation. Most desert 'seed-eating' rodents are climate 'responders' not climate 'integrators'. Desert small mammal populations exhibit large changes in abundance in relation to rainfall and food availabihty. The exception to this generalization appears to be the dominant North American heteromyid rodents (kangaroo rats and pocket-mice) (Whitford, 1976). Seed storage by heteromyids allows them to become climate 'integrators', i.e. species that maintain relatively stable populations despite environmental fluctuations. Seed-harvesting ants have the advantage of sociality that allows them to be successful utilizers of seeds in desert regions and to be 'climate integrators'. Although individual workers transport single seeds, the energy cost:energy benefit ratio is high when compared with the few seeds that a rodent could carry and the energetic costs of travel for that rodent. The lower metabolic costs for seed transport in seed-harvesting ants, allows them to accumulate seeds in their nests as a hedge against times with little or no seed production. Thus ants are the most successful environmental 'integrators' among the desert granivores.


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8.7 IMPACTS OF RODENTS AND ANTS ON ECOSYSTEM STRUCTURE AND PROCESSES A long-term study initially established to examine competition between seedeating ants and rodents, has produced a rich data set that provides important insights into the impacts of seed consumption on the structure of a desert ecosystem (Brown and Davidson, 1977; Brown and Heske, 1990; Heske et al, 1993). The study was designed with plots from which ants and rodents were excluded, plots from which rodents were excluded, plots from which ants were excluded, plots from which large-body size rodents were excluded, and plots that excluded neither group of granivores. The results from the first few years of the experiment showed that kangaroo rats preferentially removed large seeds resulting in a shift in the abundance of plants that produce small seeds. Ants removed more small seeds than large seeds producing a complementary effect on the vegetation in the plots (Brown and Davidson, 1977). A summary of the data after the experiment had been in place for 10 years showed that there was no evidence for either competition or facilitation between the granivorous rodents and granivorous ants based on the densities of these taxa on the experimental plots (Samson et al., 1992). Examination of the plots after 15 years showed that the interactions between ants and rodents were indirect and mediated through the vegetation rather than by direct competition. The most notable result was the positive correlation between abundance of one seed-eating ant species (Pheidole rugulosa) and the percentage grass cover on the experimental plots (Valone et al, 1994). An analysis of the long-term trends in vegetation on these experimental plots led to the conclusion that kangaroo rats (genus Dipodomys: Heteromyidae) were functioning as a keystone guild in North American desert grasslands (Brown and Heske, 1990). The kangaroo rats were shown to affect the density and species composition of the grasses in the experimental plots, especially the tall tussock-forming grasses. These data were interpreted as one explanation for changes in vegetation resulting from desertification processes. The mechanisms invoked for the effects of kangaroo rats on the grass component of the vegetation was granivory and/or soil disturbance (Heske et ai, 1993). However, these mechanisms were shown not to be feasible for the vegetation changes that were recorded over the long term of the study (Kerley et al., 1997). The kangaroo rat behavior of cutting the flowering tillers from tussock grasses appears to be a more logical mechanism for the vegetation trends recorded by Brown and Heske (1990). Characteristically cut grass tillers were absent from all rodent and medium-size kangaroo rat exclosures. Tillers cut by rodents were abundant in large-size kangaroo rat and rabbit exclosures (Fig. 8.9). This indicated that medium-size kangaroo rats (Dipodomys ordii, D. merriami) were responsible for cutting the flowering tillers. Kangaroo rats exhibit preferences for tillers of certain species of grasses. Dipodomys spp. cut very few tillers from Lehmann lovegrass (Eragrostis lehmanniana), the species that exhibited increased cover on plots with kangaroo rats. Because kangaroo rats preferentially cut flowering tillers of potential competitors of Lehmann lovegrass.

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Figure 8.9 Flowering tillers of a desert grass, Sporobolus spp., cut by rodents in the Chihuahuan Desert, New Mexico.

the increase in E. lehmanniana was attributed to the competitive advantage of this species over other tall grasses as it was subjected to less tiller pruning (Kerley et aL, 1997). Because tillers are pruned from the plants before the seeds are mature, tiller pruning must have a dramatic effect on the soil seed bank of preferred plant species. Grass tiller and seed harvesting are not the only rodent activities that have implications for vegetation structure and productivity. Heteromyid rodent cache pits are traps for seeds and organic matter and are safe germination sites for some plant species. Burrows, runways and fecal deposition areas affect heterogeneity in soil seed distribution, soil nutrient concentrations, and organic matter turnover. Soil disturbance by the degu (Octodon degus) was concluded to be the primary factor opening areas under shrubs to colonization by competing plant species and by exotic herbs and facilitating the establishment of some annuals that are restricted to nutrient rich areas under shrubs (Gutierrez et ai, 1997). There are several important lessons to be learned from the extensive and intensive studies of granivory in deserts. Obviously long-term experiments are required to provide an accurate assessment of impact of animal consumption on ecosystem structure and/or ecosystem processes. The long-term experiment clearly showed that neither the hypothesized indirect competition nor facilitation occurred between ants and rodents. The most important effect of ants and rodents


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on vegetation may be by establishment of soil fertility gradients around ant colonies or rodent burrow systems, or by the production of safe germination sites. Granivory is not the only direct consumption effect on vegetation as shown by the studies of gramnivory by desert seed-eating rodent species. Although the rodents consume very little of the grass stem (eating only a small portion of the base of the cut tiller), they contribute a large quantity of high nutrient content litter to the plant detritus pool. These studies document the complexity of what superficially appear to be simple effects of consumers on ecosystems. 8.8 VERTEBRATE PREDATORS The most abundant vertebrate predators in most desert ecosystems are lizards. Not all lizards are predators; some like the North African Uromastix spp. and the desert iguana, Dipsosaurus dorsalis, are herbivores. There are important differences in the relative abundance and species richness of lizards in different deserts of the world. Algerian Sahara lizard communities vary from five to seven species in three families depending on habitat characteristics (Grenot and Vemet, 1972). There are 18 species representing four families of lizards reported for the entire Saharan region (Lambert, 1984). The Saharan lizard communities are depauperate in comparison with lizard faunas of other deserts. Lizard communities in North and South American deserts have fewer species than deserts in southern Africa and Australia. It has been suggested that these differences reflect the paucity of termites in North American deserts in comparison to the rich termite fauna of southern Africa and Australia (Pianka, 1985). An intercontinental comparison shows that each of these areas supports five families of lizards but only one family is found in all three areas. North American deserts and the Kalahari support 12 genera and 13 genera, respectively, whereas Australian deserts support 24 genera. At the species level these differences are magnified. North American hot desert sites have six to eleven species of lizards depending on the complexity of the vegetation and landscape position, the Kalahari between 11 and 17 species depending on the site and in the Great Victoria Desert of Western Australia lizard species richness varied from 15 to 42 species (Pianka, 1985). Desert lizard communities have been divided into two general classes: widely foraging or sit and wait predators (Huey and Pianka, 1981). Sit and wait lizards tend to feed on active prey species that are moving through the environment. The prey taken by sit and wait predators are mostly lepidopterans, coleopterans, and orthorpterans (Huey and Pianka, 1981). Sit and wait predators divide space and time, foraging at different times of day and choosing different microhabitats to sit and wait. The energetics of predation by a widely foraging lacertid lizard, Eremias lugubris, and a sit and wait species, Eremias lineoocellata, in the Kalahari Desert were found to differ considerably in terms of energy expenditure and foraging efficiency (Nagy et al, 1984). Energy expenditure during the active period was 12 times resting in E. lineoocellata and three times resting in E. lugubris. Foraging

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efficiency defined as metabolizable energy gained while foraging divided by total energy spent while foraging was higher in the wide forager (2.0) than the sit and wait predator (1.6). E. lineoocellata was foraging for much shorter periods (2.75 h day~^ vs. 10.24 h day~^) than E, lugubris. Although the sit and wait predator made a greater profit on its energy expenditure (46% vs. 36%), the wide-foraging E. lugubris grew nearly twice as fast as E. lineoocellata (Nagy et al., 1984). These apparent contradictions based on energetics demonstrates that variation in food availability and/or differences in predation rate introduce variables not accounted for in the energy-based model. Energetic studies comparing sit and wait and widely foraging lizards provide some interesting insights into secondary production by predators (Anderson and Karasov, 1988). The wide-foraging North American lizard, Cnemidophorus tigris (Teiidae) and the ambush forager, Uta stansburiana (Iguanidae) did not differ in the proportion of the energy budget expended on reproduction but the total reproductive effort (which includes metabolism associated with reproduction) was lower in C. tigris. C. tigris produces smaller clutches of larger eggs than U. stansburiana but the number of eggs laid per unit time was equal for these species. It was concluded that the higher rates of production allow C. tigris to reach larger size than sit and wait iguanid lizards at the same age (Anderson and Karasov, 1988). Daily maintenance energy expenditures of widely foraging lizards appear to be about 1.3-1.5 times greater than those of sit and wait foragers. However, food gains of widely foraging lizards have been estimated at 1.3-2.1 times greater than sit and wait foragers (Huey and Pianka, 1981). Prey selection by lizards is largely a function of foraging mode. Widely foraging lizards tend to forage in patches that support large numbers of relatively sedentary prey. The prey taken by widely foraging lizards in North America, the Kalahari, and Western Australia is predominantly termites. The availability of termites at the surface is unpredictable in time and space. This requires a predator to move rapidly through the environment, testing potential patches until a rich patch is found. In North American deserts, teiid lizards {Cnemidophorus spp.) are the widely foraging lizards. Termites are reported to make up nearly 30% of the stomach contents of Cnemidophorus tigris, and 17% by volume of the stomach contents of Uta stansburiana, a moderately wide foraging species (Huey and Pianka, 1981). Termites accounted for 4% or less by volume of the stomach contents of sit and wait predators. In spinifex grassland habitats of central Australia, the diverse lizard fauna is thought to have developed with the diversity of termites in these grasslands. It is argued that the species-rich termite fauna utilize the spinifex grassland in a number of ways. This diversity of foraging behaviors in the termites allows for coexistence of a number of lizard species using termites as their primary prey (Morton and James, 1988). In a study of five species of the skink genus Ctenotus in central Australia, the high dietary overlap in these species was accounted for by the high proportion of termites in the diet (James, 1991). However, each species


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of Ctenotus ate different genera or foraging guilds of termites. During a relative dry year, termites accounted for a larger portion of the prey items than during a relatively wet year. Other prey were Araneida (spiders), Blattodea (cockroaches), Chilopoda (centipedes), Coleoptera (beetles), Formicidae (ants), Hemiptera (bugs), Orthoptera (grasshoppers and crickets) and Thysanura (silver fish). Based on this two-year study, James (1991) concluded that Ctenotus spp. lizards were somewhat opportunistic in prey selection and that the abundance of prey has a large effect on hzard diets. A relationship between rainfall (with a one year lag) and population size of lizards has been documented and related to prey abundance (Whitford and Creusere, 1977). A relatively long-term study of desert lizard communities related changes in biomass, secondary production and diversity to annual precipitation and growing season precipitation (Whitford and Creusere, 1977). There were differences in the responses of lizards in two different landscape units on a watershed. The species richness of the lizard communities ranged from 6 to 11 species in a grassland-shrub mosaic at the base of the watershed and from six to eight species in a creosotebush (L. tridentata) shrubland on the upper bajada (piedmont). Both communities were dominated by active foraging species (Cnemidophorus spp.). Recruitment of juveniles was virtually absent during the late summer and autumn of years with below-average summer rainfall. The recruitment of juveniles was extremely high during an exceptionally wet growing season. As a result of two successive 'above-average precipitation' growing seasons, several species of hzards immigrated into areas that they had not occupied during average or belowaverage rainfall years. During a drier-than-average growing season following the wet years, the immigrant species disappeared in the creosotebush community but remained present in the grass-shrub mosaic habitat at the base of the watershed. These differences in the responses of lizards were attributed to productivity differences in the landscape resulting from soil water storage and retention. Lizard species that immigrated into marginal habitat originated from more mesic mountain slope areas and toe-slope grassland. The total linear distance from these source habitats to the base of the watershed was approximately 3 km. Immigration is less likely to be a factor affecting the species richness and productivity of lizard communities in habitats that are distant from source areas (Whitford and Creusere, 1977). These responses of desert lizard communities were summarized in a conceptual model (Fig. 8.10). The relationships of desert lizard populations and rainfall can be complicated by interspecific competition or changes in vegetation cover. For example, the Australian netted dragon, Ctenophorus nuchalis was most abundant when the spinifex bunch grass {Triodia basedowii) was less then 10% cover. Another dragon, C. isolepis, reached numerical dominance following heavy rains that increased spinifex cover to greater than 20% (Dickman et ai, 1999). It was suggested that drought-wet cycles periodically reverse the dominance of the two species of Ctenophorus and probably of other species. This pattern of dominance shift is similar to that reported for two species of kangaroo rats, Dipodomys spp..

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Ecology of Desert Systems Growing Season Precipitation

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in the Chihuahuan Desert of North America (Whitford and Steinberger, 1989). Although the community dominance may change, the biomass responses of the lizard assemblage remains the same as proposed by Whitford and Creusere, 1977). Although the division of lizard communities into two 'foraging guilds' has been useful in distinguishing between those species that feed predominantly upon termites and those species that take whatever comes by, it does not account for species that are not 'sit and wait' but move slowly and deliberately from patch to patch. This is the foraging behavior of lizards that are ant specialists. Although ants are the most abundant surface-active arthropods, most of the biomass of an ant is indigestible chitin. Lizards that specialize on ants as a food source share some important behavioral traits. Several studies of the North American homed lizards (genus Phrynosoma) have examined the foraging behaviors of a deliberate slow-moving predator. Homed Hzards are ant specialists. The juvenile and adults of a relatively large species, Phrynosoma cornutum (Fig. 8.11), preys almost exclusively on large harvester ants of the genus Pogonomyrmex (Whitford and Bryant, 1979). A small congener, Phrynosoma modestum (Fig. 8.12), feeds almost exclusively on honey-pot ants of the genus Myrmecocystus (Shaffer and Whitford, 1981). P. cornutum remains in one place as long as the time span

Figure 8.11 Mexico.

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Figure 8.13

Thorny devil, Moloch horridus, central Australian Desert.

between individual ants appearing within the capture space does not exceed some threshold value (Munger, 1984). P. cornutum seeks the canopy of subshrubs as a resting place during the heat of the day when ants cease foraging. By cUmbing into the canopy of a small shrub, this lizard can maintain its body temperature at 40°C, which allows it to digest the nonchitinous parts of its prey. There is morphological and behavioral convergence in North American horned lizards and a species of AustraUan agamid Uzard, Moloch horridus (Fig. 8.13), which is also an ant speciaHst.

8.9 BELOW-GROUND FOOD WEBS The soil fauna in deserts is important as detritus consumers and as regulators of decomposition and nutrient cycling. The soil macrofauna in warm deserts is dominated by subterranean termites. However, not all warm deserts have subterranean termites (Crawford et a/., 1993). At the southern margin of the Chilean arid zone, there is evidence that earthworms dominate the soil macrofauna. Earthworms have also been reported from the wetter margins of the Sahara. Earthworms are probably limited to areas with silty soils where soil moisture remains relatively high either from rainfall or from fog condensation (Crawford et al., 1993). In most arid regions termites dominate the macrofauna. The termite faunas of Africa


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and Australia are the most diverse in the world and include a number of mound building and subterranean species. High biomass of subterranean and epigeic termites have been reported from Saudi Arabia and high species richness has been reported from the arid regions of Africa and the Arabian Peninsula (Johnson and Wood, 1980; Badawi et al, 1984). Although the termite fauna of North American deserts is depauperate, the dominant species are generalists that consume most dead plant materials and animal dung (Whitford, 1991) Desert soil mesofauna is dominated by mites (Acari) of the suborder Prostigmata with some mites in the suborder Cryptostigmata in microsites with sufficient litter layers. In mesic environments and semiarid woodlands, cryptostigmatid mites dominate the soil microarthropod assemblages (Kinnear, 1991). The prostigmatid mites in desert soils tend to be generalists that feed on fungi or on nematodes or both (Whitford, 1996). Other microarthropods that are abundant in some microhabitats include springtails, Collembola, book-lice, Psocoptera, and a variety of insect larvae (Wallwork, 1982). Most of these taxa feed directly on dead plant material and/or fungi. There are many similarities in the microarthropod faunas of the deserts of the world (Noble et al., 1994). Many genera of soil mites are shared among North American deserts, Australian deserts. South American deserts and deserts of southern Africa (Crawford et al, 1993; Noble et al, 1994). The diversity and abundance of microarthropods varies with soil type and vegetation. In shrub deserts, microarthropods are abundant under shrub canopies where litter accumulates (Santos et a/., 1978). Soils under shrubs in more mesic microhabitats support higher densities and diversities of microarthropods (Wallwork et aL, 1985; Crawford ^^

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fragmentation by rainfall. Breakdown by microbial enzymes is obviously limited to those brief periods when litter and soil are moist and soil temperatures are moderate. Biological decomposition of surface materials is therefore sporadic and brief. Measurement of mass loss cannot distinguish between fragmentation and mineralization. However, if mass loss is a function of biological activity, there should be a relationship between frequency and duration of litter wetting and mass loss since growth and activity of microorganisms requires moist environments. Experiments designed to examine this question by providing supplemental water by sprinkler irrigation have shown that there is no clear relationship between quantity and frequency of rainfall and mass loss (Whitford et ai, 1986; Mackay et aL, 1987; Steinberger and Whitford, 1988). Adding 305 mmyr'^ to the 230 mm annual total had no significant effect on mass losses nor did frequent pulses (Whitford et aL, 1986). When Utter was placed in the field during an extended dry period, irrigated Utter lost mass faster than nonirrigated Utter. The rates, however, were equal thereafter (Fig. 9.2). Further insights into this process were gained in irrigation experiments in the Negev, Israel. In the Negev, the probabiUty of rainfall in the summer months of May-September is essentially zero. Although

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irrigation added considerable water and kept soils and litter wet for longer periods, there were no detectable irrigation effects during the normal winter-spring 'rainy period'. Irrigation increased mass losses during the rainless summer (SteinbergerandWhitford, 1988). Although rainfall supplementation experiments provide some insights into how climate variation affects decomposition, imposition of drought is essential to really understand climate-decomposition relationships. 'Rain-out' shelters constructed of steel frames with greenhouse plastic roofs were used in several studies in the Chihuahuan Desert. These studies demonstrated that the imposition of drought resulted in a marked decrease in average litter inputs (45 g per trap compared to 83 g per trap in year 1, and 72 g per trap compared to 112 g per trap in year 2 in droughted plots and controls, respectively). Imposition of drought had no effect on the rates of decomposition of creosotebush leaf litter on the soil surface (Fig. 9.2). However, decomposition rates of roots of a perennial herb were significantly reduced by imposed drought but not by irrigation (Fig. 9.2). 9.2.3 Spatial Variation Variation in microclimate and physicochemical soil properties resulting from topographic position and vegetative cover can produce different patterns of decomposition, mineralization and soil organic matter accumulation (Seastedt, 1984). In a study of surface litter decomposition at several locations on a desert watershed, there were no differences in initial rates of mass loss (k^) across the range of locations on the watershed. There were differences in average rates of mass loss among locations in the second phase of decomposition (fc^). Highest rates of decomposition were recorded for a run-on area at the base of the watershed. The lowest rates of decomposition were recorded in the dry lake basin on fine textured clay soils (Cepeda-Pizarro and Whitford, 1990). The high rates of decomposition in the run-on area were attributed to partial or complete burial of the litter bags by water-transported soil. The low rates of decomposition measured in the dry lake basin were attributed to the extremely low infiltration of water into those soils and shading by the dense grass cover. In contrast to surface litter, decomposition of roots on the same watershed yielded no significant differences among sites, except for the dry lake basin, on the same watershed where the surface litter studies were conducted. There were no significant differences in decomposition rates of roots in nitrogen fertilized soils and in unfertilized soils (Mun and Whitford, 1998). The spatial differences in mass loss rates in the dry lake were attributed to the high clay content soils of the lake basin which reduced water availability and to the absence of termites in the lake basin. Therefore, it appears that topographic position and soil textural characteristics have little effect on decomposition processes except when landscape position results in litter burial or where fine texture soils affect the composition of the soil fauna.


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9.2.4 Decomposition of Buried Litter and Roots Litter that is buried in the soil decomposes more rapidly than surface litter and is primarily a biological process (Santos et ah, 1981; Elkins and Whitford, 1982; Parker et ai, 1984). Decomposition rates of buried litter varies inversely with initial Hgnin content and C:N ratio. The chemical changes in decomposing buried htter are the same as those reported for forest litter (Berg et al, 1984). The early stages of buried litter decomposition involves losses of soluble materials and holocellulose but essentially no change in the lignin. Nitrogen immobilization occurs in the early stages of buried litter decomposition as the microbial biomass growing on the Htter increases (Parker et al., 1984). The biotic regulation of decomposition of buried materials was clearly documented in studies in the hyperarid Namibian sand sea, southwestern Africa. Decomposition of cellulose (cotton cloth and filter paper) buried at 10 cm depth occurred in pulses following rain events. About 84% (range 64.7-97.2%) of the original mass was lost following rains of 9 mm or greater (Jacobson and Jacobson, 1998). Mass loss during a dry period was only 8.2% (range 0-16.7%). Most substrates were colonized by fungi. Termites and tenebrionid beetle larvae were recorded as feeding on the fungus-colonized substrates. In hyperarid systems where soils are too dry to support growth of fungi and activity of microarthropods, rainfall is the trigger that stimulates episodic periods of high rates of decomposition. The episodic pulses of decomposition result from rapid growth of fungi and activity of soil mesofauna and macrofauna that feed on fungi and fungal modified cellulose. Considering that the recalcitrant materials in surface Utter are rapidly destroyed by photodecomposition and that only a fraction of the Htter is buried, it is obvious that the largest contribution to the recalcitrant soil organic carbon pool is that of decomposing roots. Roots vary considerably in initial Hgnin content (approximately 17% for roots of an annual plant, 24% for roots of a woody shrub, and 27-29% for roots of several grasses. Decomposition rates of roots were inversely related to the initial lignin content and also to the initial nitrogen content which were lower in the roots of grasses than in the other species examined (Mun and Whitford, 1998). Decomposition rates of roots in the Chihuahuan Desert were equal to or higher than those reported for decomposing roots in mesic ecosystems (Whitford et al, 1988). Rates of decomposition of roots of woody shrubs and of herbaceous annuals were not affected by supplemental irrigation. The absence of an irrigation effect was attributed to the relatively 'average' rainfall during thetimethat decomposition was measured. Average annual mass losses from woody roots (40%) were considerably lower than the average annual mass losses from roots of herbaceous annuals (85-90%) which was related to the Hgnin content of the roots and to the susceptibility of herbaceous roots to attack by subterranean termites. Simulated drought reduced the mass loss from roots of an herbaceous perennial from an average mass loss of 80% to an average mass loss of 50% (Whitford et al, 1995; Fig. 9.3). The roots of the woody shrubs retained relatively constant C:N ratios through the experiment indicating Httle if any nitrogen immobiHzation. The C:N ratios of the roots of

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annuals decreased by an average of 28 after 4 months indicating that microbial biomass on the roots of the annuals had immobilized a large amount of soil nitrogen. The changes in carbon chemistry of decomposing roots in a desert are similar to those of decomposing roots in mesic forests and in mesic grasslands (McClaugherty et ai, 1984). The percentage composition of nonpolar compounds and watersoluble compounds in decomposing roots of all species dropped to approximately 20% of their original concentration within the first year (Mun and Whitford, 1998). The rapid decrease in these compounds is probably the result of rapid colonization of roots by bacteria and fungi and the subsequent rapid metaboHsm of these easily metabolizable compounds. The acid-soluble compounds are more recalcitrant than


247

the nonpolar and water-soluble compounds and these decreased by only 10% of their original concentration at the end of thefirstyear and to between 10% and 25% (depending on the species) after 42 months of decomposition. The relative concentration of lignin increased during the decomposition of roots and there were marked increases in the relative concentration of nitrogen in the roots of an annual but nitrogen immobilization in decomposing roots of woody plants did not occur until after 24 months of decomposition (Mun and Whitford, 1998). The evidence from changes in chemistry of decomposing roots and of buried litter supports the conclusion that decomposing roots are the principal source of recalcitrant carbon in desert soils. 9.2.5 The Role of Microfauna in Decomposition and Mineralization The soil microfauna have been shown to play a major role in decomposition processes in mesic temperate ecosystem (Swift et al, 1979; Seastedt, 1984). The soil microfauna in arid systems is nearly as diverse as the soil microfauna of mesic soils (Wallwork, 1982). In order to adequately address the role of soil biota in decomposition, it is necessary to distinguish between the plant litter, dung, wood, etc., exposed to the intense light and heat of the soil surface and the litter, litter fragments and dead roots at varying depths below the soil surface. Organic materials on the soil surface have microclimatic conditions conducive to microbial growth and activity of soil microflora for brief periods at night and for sHghtly longer periods following rains (Whitford, 1989). However, the micromesofauna appear to have virtually no effect on the rates of mass loss from surface litter (Silva et aL, 1985; MacKay et aL, 1987). Some plant litter is buried in pits produced by animals and an unknown fraction of the surface leaf litter is mixed into the surface soil as small fragments. The microclimate in the soil, even as shallow as 1 cm depth, is very much modified compared to surface conditions. The moderate soil temperatures and high humidity in the soil interstices, produces microclimates that allow some microarthropods to remain active even in soils with water potentials lower than -6.0 MPa (Whitford, 1989). In soils with water potentials lower than -3.0 MPa most of the soil protozoans are encysted, the nematodes are in a state of anhydrobiosis as are the collembolans and some of the soil acari. The active mesofauna in dry soils are soil acari. There are some species of soil acari that remain active in soils as dry as -6.0 MPa and some species that enter a cryptobiotic (inactive) state in dry soils (Whitford, 1989). In desert soils the complete soil micro- and mesofaunal community is involved in decomposition and mineralization only during periods when the soil is moist. Rainfall regulates decomposition and mineralization in arid ecosystems by stimulating rapid growth of fungi and activating the soil fauna. Rainfall-induced pulses of activity are observed most clearly in hyperarid systems where rainfall events are separated by sufficient time periods for soil to dry between events (Jacobson and Jacobson, 1998). The importance of the soil microarthropod fauna in mass loss of buried litter in deserts has been well documented (Santos and Whitford 1981; Santos et aL,

248


1981; Elkins and Whitford, 1982; Parker et al, 1984). In desert ecosystems, the activity of soil microarthropods (primarily soil acari), accounted for 30% of the initial mass losses. Despite the fact that the amount of soil respiration that can be attributed to all soil animals is 10% or less than the total amount (Reichle, 1971). The soil fauna directly or indirectly influence the soil microflora by their feeding activities. The effects of soil acari on decomposition in arid ecosystems appear to be indirect. The common soil acari that are active at high soil water potentials are small prostigmatid mites that feed on fungi and on nematodes. In the initial stages of decomposition of buried creosotebush leaf litter, microarthropods controlled the population densities of bacterial feeding nematodes. Where microarthropods were excluded, the high nematode populations reduced the bacterial biomass resulting in reduced rates of mass loss (Santos et ai, 1981) (Fig. 9.4).

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249

Whereas soil bacteria dominate the early stages of buried leaf litter decomposition, the later stages are dominated by fungi (Parker et al, 1984). The microarthropods that are most abundant in buried decomposing litter or around decomposing roots are small fungal feeding mites. Although microarthropods have very little effect on mass losses during the later stages of decomposition, i.e. after the first 90 days they do directly affect rates of nitrogen mineralization. 9.2.6 The Role of Termites Termites are abundant in the soils of most subtropical, arid, and semiarid regions of the world (Lee and Wood, 1971). Although large above-ground mounds are an impressive indication of the presence of these animals, the majority of termite species live below the soil surface. Based on the distribution of termites and the limited studies of the quantities of dead plant parts and other organic materials consumed by these animals, it is clear that these insects have a significant effect on nutrient cycling and nutrient availability in arid and semiarid systems. Termites are responsible for removing a large fraction of certain species of litter (Silva et al, 1985; MacKay et aL, 1987). Soil moisture and temperature affect termite activity, hence mass loss from litter species consumed by termites shows a strong positive correlation to rainfall (MacKay et aL, 1987). Materials such as grass leaves and annual plant leaves and stems that are preferred foods by Chihuahuan Desert termites are harvested during periods when soil moisture and temperature allows termites to forage on surface litter (Whitford et aL, 1982; MacKay et aL, 1986). For example, Gnathamitermes tubiformans cease foraging at soil moisture less than -5.4 ± 0.2 MPa and hence are capable of feeding in litter accumulations for much of the year when soil temperatures are higher than 5°C (MacKay et aL, 1986). In southeastern Australia, termites were harvesting or directly consuming grass leaves and fragments, shrub and tree leaves and phyllodes, sheep and kangaroo dung, dead wood, fungal modified stem bark, and dead roots (Whitford et aL, 1992). In southern Somalia, litter decomposition was rapid (95% disappeared in one year) and not correlated with rainfall because termites were active throughout the dry season (Thurow, 1989). In the northern Chihuahuan Desert, mass loss from plant litter on plots from which termites had been chemically excluded were compared to mass loss from plant litter on plots with termites present. These studies demonstrated that a large fraction of dead plant parts and dung were consumed by termites (Whitford et aL, 1982, 1988) (Fig. 9.5). Materials ingested by termites are mineralized by the symbionts within the gut of the termite. The breakdown of materials within the termite gut includes not only the cellulosic compounds, but also lignins (Lee and Wood, 1971). The small quantities of fecal material produced by termites are generally used in the cementing materials of gallery walls and nest chamber walls. The mineral nutrients in the food material are largely retained in the biomass of the colony and are returned to the ecosystem only via predators of the workers and reproductives

250

/


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Livestock and rabbit dung breakdown A40-50% yr"^

Dead leaves and annual plant parts AIO-50% yr"^

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Yucca inflorescence decomposition A6% yr"^

Dead grasses AIO-90% yr

Figure 9.5 The keystone role of subterranean termites on decomposition processes and ecosystem function in Chihuahuan Desert ecosystems.

(Schaefer and Whitford, 1981). Because so much of the decomposition of organic matter in arid ecosystems occurs within the guts of termites, there appears to be a negative correlation between termite abundance and soil organic matter or soil nutrients. On a Chihuahuan Desert watershed where measurements of termite abundance and activity and soil organic matter were made at 30 m intervals over a 2 km transect, there was a highly significant correlation, r - -0.97 between termite abundance and soil organic matter (Nash and Whitford, 1995). The absence of a pattern of regularly increasing soil organic matter from the top to bottom of the watershed added further support to the contention that subterranean termites were primarily responsible for the variations in soil organic matter on that watershed. The most important macro-nutrient, nitrogen, tends to vary directly with soil organic matter (Whitford et al, 1987). The negative relationship between termites and soil organic matter suggests that soil fertility varies inversely with termite


251

abundance. However, the importance of termites as producers of macropores and their effects on infiltration of water is probably more important for sustaining the productivity of desert ecosystems than their effect on reducing the nitrogen content of desert soils. 9.2.7 Conceptual Model of Decomposition Processes in Deserts The following conceptual model is proposed to account for the relationships among variables that have an effect on the decomposition processes in deserts. Initial high rates of mass losses from litter on the surface result from leaching of low-molecular-weight materials from leaf surfaces and by the breakdown of lignins by photochemical decomposition. This accounts for the direct correlation between initial lignin content and decomposition. At low light intensities and low sun angles, mass loss rates are low and occur primarily from low level activity of microorganisms. At high light intensities, photooxidation of complex structural molecules results in fragmentation by wind or water drops. The small fragments are transported into the soil by water where they are then decomposed by biological activity of the microflora during periods when soils are moist. This is the source of elevated CO2 production measured when soil and litter are moist (Parker etal, 1983). A major component of the high rates of decomposition of surface litter is the consumption of many of these materials by termites. Termites consume more than 50% of the total surface htter and may consume up to 70-80% of the litter and most of the animal dung. Materials consumed by termites are virtually completely converted into carbon dioxide and water because the symbiotic gut flora of many termites is capable of processing recalcitrant organic molecules such as lignins. Thus litter falling to the surface of the soil in deserts contributes little to the soil organic carbon pool and minerals in litter are returned to the soil in a mineralized state largely through the feces of animals that prey on termites. A variable quantity of the litter input in a desert ecosystem may be blown into soil pits and buried. The process of decomposition of buried litter and dead roots is very different from that of surface litter in arid environments. The moderate environment of the soil affects both decomposition rates and patterns of decomposition. Buried litter accumulates moisture and remains moist even in dry soils. The moderate temperature and moisture content of buried litter is conducive to growth of microflora and microfauna. Dead roots encounter the same moderate conditions in soil and serve as ready substrates for growth of the microflora. Increasing populations of microflora on decomposing materials attract microfloral grazers (protozoans, nematodes, and microarthropods), which in turn attract predators (predatory nematodes and nematophagous mites). Complex food webs are quickly established around moist roots and buried litter accumulations. The extracellular enzymes of the microflora rapidly decompose the water-soluble and nonpolar compounds (sugars, fats, waxes, starches, and cellulose). The complex acid-soluble compounds are slowly attacked by a small subset of the microbial

252


heterotrophs. Decomposition of buried litter and roots is primarily a biological process.

9.3 LANDSCAPE PATTERNS OF NUTRIENT DISTRIBUTION One of the best-documented spatial patterns of nutrient distributions in arid ecosystems are the 'islands of fertility' associated with shrubs and trees (Garcia-Moya and McKell, 1970; Charley and West, 1975, 1977; Barth and Klemmedson, 1978; Parker et aL, 1982; Virginia and Jarrell, 1983; Schlesinger et ai, 1996). The accumulation of nutrients in the surface soil beneath shrubs and the lower nutrient levels in soil between shrubs results from a whole suite of processes: plant-nutrient uptake, litter fall, dust interception, soil-erosion, soil deposition, soil biota-mineralization, decomposition, animal decomposition, fecal decomposition, and predation. The relationship between shrub morphology and spatial pattern on below-canopy litter accumulation has been documented for one species but undoubtedly applies to a number of other species of shrubs (DeSoyza et aL, 1997). In landscapes composed of patches with a variable mix of shrubs, grasses, and perennial herbs, the nitrogen stores in the soil are related to the characteristics of the patch. Available nitrogen was considerably higher in soils from complex patches, intermediate in simple shrub-grass patches and lowest in patches of bare soil (Mazzarino et aL, 1996). Although the activity of subterranean termites may be enhanced by the presence of leaf litter and the moderating effect of the canopy on soil temperature, shrub and tree leaves rank low on the food preference scale of most termite species. Tree and shrub leaf litter, therefore, provides the energy and nutrient source for the soil microbiota, resulting in nutrient enrichment. Another factor affecting the nutrient enrichment of the subshrub soil is the activity of animals like lizards. Many lizards move quickly from shrub to shrub spending most of their time foraging or simply sitting in the more benign subcanopy environment (Creusere and Whitford, 1982; Peterson and Whitford, 1987). In the northern Chihuahuan Desert, lizards are the most important predators of subterranean termites. In other arid regions, e.g. Australia and the Kalahari of southern Africa, lizards are the most important termite predators. Many lizards feed almost exclusively on termites (Pianka, 1985; James, 1991). In these deserts, lizards use clumps of vegetation as shelter and as foci for their foraging activities thereby contributing to the 'islands of fertility'. Given the abundance of termites and lizards in arid and semiarid regions, it is likely that this linkage between detritivore-predator and spatial nutrient pattern is generally important in all such systems. The general pattern of nutrient distribution with high concentration in center to mid-canopy soils decreasing rapidly from mid-canopy to canopy edge and into the intershrub space, is characteristic not only of shrubs, but also of perennial grass clumps and small trees. Nutrients are accumulated where soil organic matter is elevated either by debris accumulation or turnover of fine roots.


253

Fossorial mammals, burrowing mammals, mammals and lizards that dig for invertebrate prey contribute to the nutrient heterogeneity of soils in arid regions. Foraging pits fill with litter, frass, dung, and seeds creating small nutrient-rich 'hot spots'. Burrow systems that are occupied by successive generations of central place foragers, e.g. banner-tail kangaroo rats, hairy-nosed wombats, harvester ants, and mound building termites) gradually accumulate nutrients in the mound soils and eventually develop into nutrient rich patches (Fig. 9.6). Fossorial mammals turn over soil and leave patches that are rich in organic matter in storage chambers and defecation sites (Whitford and Kay, 1999; Whitford, 2000). The abundance, species composition, and behavior of those animals that disturb soil or accumulate organic materials in burrows are important variables to consider when assessing the patterns of nutrient distribution across the landscape. The spatial distribution patterns, morphologies, and physiological characteristics of dominant plant species clearly affect the richness of the below canopy 'islands of fertility'. However, when the frame of reference is moved from the local patch to the landscape scale there are many other variables that affect the nutrient status of soils and nutrient cycHng in the ecosystems that make up a landscape. One of the most important variables that affect both nutrient status and the temporal characteristics of nutrient cycling is the spatial distribution of different root systems in a landscape. There are some generahzations about root systems

i^'Si^^Ww** .**«'*JlW'.y^

Figure 9.6 Vigorous growth of a herbaceous annual, Erodium texanum, in a corona around the nest of a central place forager - seed-harvesting ants, Pogonomyrmex rugosus. Nitrogen concentrations in the soils of the corona were double that of the surrounding soil.

254


that can be inferred from the above-ground Ufe form of the vegetation. Perennial grasses tend to produce a dense but relatively shallow mat of roots below the basal area of the plant. The root system of some species may occupy an area somewhat larger than the canopy crown of the grass but few grasses have spatially extensive root systems. Some species of grasses have physiological characteristics that result in complex rhizosheaths of mucilages and mucigels, and microbes (Wullstein et al., 1979). In some species the mucilages and mucigels secreted from the roots of grasses serve to attract and support free-living nitrogen fixing bacteria in the rhizosheath (Mandimba et al, 1986; Reinhold et ai, 1986). The root systems of most herbaceous annuals may consist of a main tap root with fine roots emerging from the bottom third or of three or more roots that extend from the root crown. Herbaceous annuals that germinate during the growing season may have shallow root systems and root: shoot ratios as low as 0.2. In herbaceous annuals that germinate during the nongrowing season (i.e. winter-spring annuals in North American deserts), the rooting depth and root biomass is dependent on the depth of the wetting front and the length of time that the deeper soil profile remains moist. During cool periods when aboveground growth does not occur, the roots of the rosettes continue to grow. This can produce root: shoot ratios as high as 1.2 at peak above-ground biomass production. The root biomass of annual plants can vary both temporally and among species. Perennial herbaceous plants are generally but a small fraction of the cover and biomass of desert vegetation. Some perennial herbaceous species are clonal and may occur in dense patches. Usually there is more below-ground biomass of herbaceous perennials than above-ground biomass. The root systems are generally fleshy (storage organs) with fine roots emanating from the actively growing segments. Most desert shrubs and small trees have diffuse, relatively deep (> 1 m) woody roots systems. Many desert trees and shrubs also have extensive shallow root systems in addition to the deep roots (Gile et al., 1998). In most desert soils, nitrogen is concentrated in the upper 10 cm of soil. The growth responses of a nitrogenlimited desert shrub and small rain events suggest that the shallow roots contribute most of the nitrogen uptake and allow the shrub to respond rapidly to relatively small rain events (Fisher et a/., 1987). Plants with symbiotic nitrogen fixers are common in arid landscapes. Many species of small tree and shrub legumes have nodules containing the nitrogenfixing Rhizobium spp. Not all legumes host symbiotic nitrogen fixers on the root system. Several species of legumes have increased in abundance and cover in areas where domestic stock grazing has modified the environment, e.g. Acacia caven (in Chile), Prosopis spp. (in North America) and Acacia spp. (in Africa). These species produce leaves with lower C:N values than those of other nonleguminous species even in those plants with root systems devoid of nodules. Even when a legume is removed, there is a residual elevation in soil nitrogen levels (Klemmedson and Tiedeman, 1986). This has important implications for


255

the patchiness of vegetation, species composition of the vegetation and the productivity of arid areas in which woody legumes dominate the landscape. The abundance and spatial distribution of such species affects soil nitrogen levels and productivity of grasses and other plants that grow under the canopies of woody legumes (Weltzin and Coughenour, 1990). Nutrient enrichment of soils under legume canopies can result in production values that are twice those measured in the between-canopy space (Weltzin and Coughenour, 1990). Herbaceous nitrogen-fixing legumes are a relatively minor component of the vegetation. Some of the nitrogen-fixing legumes are ephemeral plants, e.g. species such as Astragalus spp. in North America. Most of the herbaceous nitrogenfixing legumes are perennial. However, perennial herbaceous legumes accounted for only a fraction of one percent of the vegetative cover on most locations on a desert watershed (Whitford et a/., 1987). It is unlikely that herbaceous legumes contribute much to the nitrogen economy of desert ecosystems. There is a clear correlation between soil organic matter and nutrient availability in desert ecosystems. This is not a surprising correlation because the primary source of nutrients in soils is from the decomposition and mineralization of organic materials. In a study using nitrogen mineralization potential as a measure of nutrient availability to plants, Whitford et al. (1987) found good correlations between mean soil organic matter and mineralized N (Fig. 9.7). The spatial distribution of soil organic matter in deserts is determined by (1) location and relative abundance of termites, (2) run-off, run-on patterns, (3) resistance of the vegetation to wind transport of litter and (4) structure of the vegetation mosaic. There is a positive relationship between soil organic matter, total nitrogen and nitrogen mineraHzation potential (the rate of nitrogen mineralization under defined conditions of soil incubation). This relationship is well documented in the northern Chihuahuan Desert (Fig. 9.7). As the data for the dry lake bed soils show, the direct positive relationship may be somewhat modified by soil texture. Total N-organic matter relationships are also affected by the genetic physiological characteristics of the vegetation. For example, the total soil N is elevated in mesquite coppice dune soils because of the lower C:N ratio of Prosopis glandulosa leaves in comparison to other shrubs and grasses in the area. Another important consideration affecting the spatial variability of soil nutrients is the negative relationship between soil organic matter and subterranean termites. Soils that are unfavorable for termites will tend to have higher levels of soil organic matter and higher nutrient concentrations. The spatial pattern of termite colonies and their foraging patterns will certainly affect the variability in nutrient availability across an arid landscape. The morphology and physiology of the dominant plants also affect the variability and the relative importance of various processes. For example, in plants with canopies shaped as inverted cones, accumulation of dust on the leaves may contribute greatly to nutrient concentration around the root crown because of effective stem flow (DeSoyza et al., 1997). Perennial species with symbiotic Nfixersmay produce nitrogen rich patches, but the concentrations of other nutrients may not be proportioned to the N concentration.

256


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The patterns of nitrogen mineralization and soil organic matter across a Chihuahuan Desert landscape reflect these variables. The inverse correlation between termite relative abundance and soil organic matter has been well established for a Chihuahuan Desert watershed. In deserts where termites are more abundant, soil nutrient levels may be considerably lower than the concentrations recorded in North American deserts. The pattern of nitrogen mineralization on a watershed also reflected the run-off, run-on patterns of organic materials. The dry lake basin where the highest nitrogen mineralization and highest total soil N were recorded is not only enriched by organic matter run-on but is also an area where the fine textured soils and infrequent inundation eliminate termites. The relationships between soil organic matter and landscape position, soil characteristics, and nitrogen mineralization are probably applicable to most arid and semiarid landscapes.

9.4 RESORPTION An important variable that affects decomposition and mineralization is the resorption of nutrients from fohage prior to leaf abscission. In the process of resorption, nutrients are mobilized from senescing leaves and transported to other plant tissues. Nutrients that are resorbed are conserved by the plant for use in primary production in the next growing season. Nutrient resorption is especially important for plants growing in nutrient poor environments (Aerts, 1996). In a review of nutrient resorption in perennial plants, Aerts (1996) reported that mean nitrogen resorption efficiency was significantly lower in evergreen trees and shrubs (47%) than in deciduous trees and shrubs (54%). Phosphorus resorption efficiency was essentially the same for evergreen (51%) and deciduous (50%) shrubs and trees. Nutrient resorption patterns in desert plants differ from the general patterns for mesic species. Resorption of phosphorus in creosotebush (L. tridentata) was very high (72-86%) (Lajtha, 1987). However, the nitrogen resorption values for creosotebush fell within the range of values for evergreen trees and shrubs (Lajtha and Whitford, 1989). Mean resorption efficiencies calculated on the data for desert shrubs for nitrogen and phosphorus were 57% and 53%, respectively. There was a large intersite and interyear variation in resorption efficiencies of ocotillo, Foquieria splendens, suggesting that the regulation of resorption efficiencies in desert shrubs may be quite complex. Winter-deciduous shrubs growing on the margins of ephemeral stream channels exhibited nutrient resorption patterns related to their status as facultative or riparian species. Nitrogen and phosphorus resorption efficiencies of obligate riparian species {Brickellia laciniata and Chilopsis linearis) (N = 63%, P = 58%) were substantially higher than in facultative riparian species (Fallugia paradoxa, Rhus microphylla, Prosopis glandulosa, and Flourensia cernua) (N = 46%, P = 46%). The species resorbing the least nitrogen from the leaves was mesquite, P. glandulosa; a species known to harbor nitrogen-fixing symbionts on the roots

258


(Killingbeck and Whitford, 2001). It has been hypothesized that plant species with nitrogen-fixing root symbionts will resorb less foliar nitrogen than other species in the same environment. In the Sonoran Desert, no resorption of nitrogen was reported in senescing leaves ofR glandulosa (Rundel et al, 1982). 9.5 NITROGEN CYCLE 9.5.1 Nitrogen Fixation Of all of the soil nutrients required for plant growth, nitrogen appears to be the most important limiting nutrient in semiarid and arid ecosystems. Despite the huge atmospheric pool of nitrogen, that pool represents a nonusable form of nitrogen for use in photosynthesis. The biologically active forms of nitrogen are ammonium and nitrate which can be used to produce amino acids and other nitrogen-containing organic molecules. The fate of fixed nitrogen is complex (Fig. 9.8) and involves a variety of microorganisms, soil microfauna, plus plants and animals. The fixation of atmospheric nitrogen (N2) is an energy-expensive reduction to ammonium (NH^) that occurs in some cyanobacteria, in nodules on the roots of legumes by Rhizobium spp. bacteria, and in some free-living N-fixing bacteria Atmosphere (N2)

Soil N2 fixation (Symbiotic bacteria (Rhizobium & Frankia) cyanobacteria (Nostoc etc.) free living bacteria (Azobacter, Clostridium, Azospirillum) N2 + 4H* -^ NH4*

Nitrification NH4* + 2O2 -^ NO3 {Nitrosomonas and Nitrobactei) (plant uptake of N H / and NO3, incorporation into plant proteins)

Dead plant parts, animal feces (decomposition and mineralization) NH3 -^ nitrification and/or plant uptake Nitrogen Losses from Organic Pool Ammonia Volatilization N H / + OH" NH3T + H2O Denitrification NO3 -^ N2O -^ Ngt (Pseudomonas, Acliromobacter)(requ\res anaerobic or extremely low soil oxygen concentration)

Figure 9.8

A general nitrogen cycle applicable to desert ecosystems.


259

such as Azobacter spp. Some other genera of free-living bacteria such as Azospirillum sp. may fix atmospheric nitrogen but there is conflicting evidence concerning these organisms as N-fixers. Cyanobacteria are widely distributed as components of the algal-lichen (cryptogamic) crusts that are common in many deserts (Fig. 9.9). Cyanobacteria also occur in some desert soils where microphytic crusts appear to be absent. In areas with well-developed cyanobacterial crusts, atmospheric N-fixation may contribute significant amounts of nitrogen to the soils. Soils with cyanobacterial and lichen crusts had nitrogen contents 4 to 7 times higher than soils with no lichen or algal crusts (Shields, 1957). Nitrogen fixation in desert algal crusts, measured by uptake of isotopically labeled nitrogen, was rapid after initial wetting of the crusts. Cyanobacterial and Hchen crusts leak thefixednitrogen to the soil, usually in the form of ammonium, thereby increasing soil nitrogen levels (Mayland and Mcintosh, 1966). Nitrogenfixationis not the only influence of cyanobaterial and lichen crusts on soil nutrient enrichment. The concentrations of the elements N, P, K, Ca, Mg, and Fe, were significantly higher in a grass, Festuca spp. growing on cryptogamic (algal and lichen) crusted soil than on soil with no cryptogamic crust (Belnap and Harper, 1995). They reported that cryptogamic crustsfixedconsiderable nitrogen, which becomes available to vascular plants via decomposition and mineralization of the cryptogams or by cellular secretion in the living cryptogams. In deserts, the

Figure 9.9 Cryptogamic crusts covering bare patches in a tabosa grass, Pleuraphis mutica, swale in the northern Chihuahuan Desert, New Mexico.

260


nutrient enrichment of soils by cyanobacterial and lichen crusts is rainfall dependent. In an area with 15% surface cover of algal crusts, up to 1.3 g N ha"^ h"^ was added to the soil during the daylight hours for the first 24 hours after a rainfall (Loftis and Kurtz, 1980). Symbiotic nitrogen fixation in desert plants may be by Rhizobium nodules or nodules containing actinomycetes. One of the important tree species in the temperate and tropical semiarid zone of Australia is Casuarina cunninghamiana, which harbors actinomycete symbionts (Frankia spp.) in nodules on its roots. Frankia spp. on the roots of this tree fixed considerable nitrogen at soil temperatures between 20°C and 30°C (Reddell et aL, 1985). Actinomycete symbionts may be important in nitrogen fixation in other woody plant species in arid regions but there is insufficient information on most species to evaluate this possibility. Tree and shrub legumes are abundant in most arid and semiarid regions of the world. Many of the most common genera harbor Rhizobium spp. or Bradyrhizobium spp., symbiotic, nodule-forming, nitrogen-fixing bacteria. Among the common genera confirmed to have nodulation and nitrogenase activity are Acacia, Prosopis, Pterocarpus, and Pericopsis (Shearer et al., 1983; Hogberg, 1986). Studies using nitrogen abundance ratios (N^^/N^^) confirmed symbiotic nitrogen fixation in Prosopis at six of seven sites in the Sonoran Desert. Nitrogen fixation was also reported in other Sonoran Desert legumes of the subfamily Papilionoideae: Lupinus, Dalea, Astragalus and Lotus (Shearer et al., 1983). An abundant shrub (Psorothamnus spinosus) growing in the low nitrogen environment at the margins of ephemeral streams obtained approximately onethird of its nitrogen from symbiotically fixed N. From the limited available data, it appears that most of the woody legumes and a large portion of the herbaceous legumes obtain fixed atmospheric nitrogen from symbiotic Rhizobium or Bradyrhizobium species. Nitrogen fixation by root symbionts is probably the primary source of nitrogen in the fertile islands that develop under the canopies of leguminous trees and shrubs. Although nitrogen fixation in Rhizobium nodules benefits the host plant with the nitrogen enrichment most of the nitrogen gain does not enter the ecosystem until the N-rich leaves decompose. The published estimates of quantities of fixedN relative to the quantities of N incorporated into growing plant tissues are not particularly reliable. In desert ecosystems, the relative importance of the various N fixation processes can probably be assessed by the presence and cover of cyanobacterial crusts, cover of legumes known to support Rhizobium nodules, and distribution and cover of vegetation life forms that provide rhizospheres conducive to the growth of free-living N-fixers. Some desert grasses and herbaceous perennials produce rhizosheaths of soil particles bound together and to the root surface by mucilages and other complex carbohydrates (Fig. 9.10). Rhizosheaths surrounding root surfaces are several times the diameter of the root that secreted the complex carbohydrates (Rougier and Chaboud, 1985). Rhizosheaths provide an ideal habitat for a variety of soil microorganisms. Some of the microorganisms inhabiting rhizosheaths are species


261

Figure 9.10 The rhizosheath on the roots of a porcupine grass, Triodia pungens, northwestern New South Wales, Austraha.

of bacteria that are free-living nitrogen fixers (Clostridium spp., Azotobacter spp., and Azospirillum spp.) (El Shahaby, 1988). In plants that produce rhizosheaths, there appears to be stimulation of nitrogen fixation. The relationship between root secretion and nitrogen fixation by rhizosheath bacteria is an example of associative nitrogen fixation. Associative nitrogen fixation is a loose symbiosis in which the energy supplied by the plant supports nitrogen fixing bacteria that supply nitrogen to the plant. Studies of associative free-living nitrogen fixation in the rhizospheres of grasses from a desert watershed showed peak nitrogen fixation during the vegetative growth stage and seed production phenophase of the grasses (El Shahaby, 1988). Grasses grown in sterile quartz sand inoculated with bacterial cultures of species isolated from grass rhizosheaths in the field, exhibited growth related to the inoculum. Grasses grown in sterile quartz sand inoculated with Qithtr Azotobacter spp. or Azospirillum spp. exhibited higher growth rates than grasses grown with a balanced nutrient solution minus nitrogen. Grasses grown with an inoculum of both species exhibited considerably higher growth than grasses grown with a single species of N-fixer (El Shahaby, 1988). The growth stimulation produced by associative rhizosheath bacteria may be produced by growth factors (hormones or hormone-like compounds) released in addition to nitrogen. In addition to N2 that is fixed by biological fixation, there are atmospheric inputs of nitrogen in the form of dust. Dryfall inputs to desert ecosystems vary considerably depending on the proximity of the desert area to industry and population centers. In the Mediterranean region of southern Europe, long-distance transport of polluted air masses increased atmospheric wet nitrogen input by at

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least a factor of 1.6 (Loye-Pilot et ai, 1990). In the northern Chihuahuan Desert there were high values of calcium, sulfate, nitrate, and total nitrogen in dryfall during the growing season. The growing season values of sulfate and nitrogen were nearly double the annual average values. These values were attributed to the differences between directional movement of air masses during the winter months and variable direction of air masses during the season of convectional storms (growing season). Air masses passing over metropolitan areas pick up nitrogen, sulfur, and other minerals that are deposited in rainfall or as dryfall. The concentrations of nitrate and total nitrogen in dry fall ranged between 4.4 ± 2.7 and 1.4 ± 0.9 ppm in dry fall and averaged 0.23 ppm in bulk precipitation (Whitford et ai, 1997). Rainfall inputs of NO3-N that averaged 39 g ha~^ cm~^ rainfall were measured in the Chihuahuan Desert in west Texas (Loftis and Kurtz, 1980). In the semiarid chaparral of southern California, precipitation and dryfall resulted in mean annual deposition of Ca, K, NH^-N, NO3 -N, Na, Mg, and S04"-S equal to or more than equal to annual losses reported in run-off from these ecosystems (Schlesinger^^t^/., 1982). 9.5.2 Nitrogen Losses Ammonia volatilization and denitrification are mechanisms of loss of nitrogen from soil pools back to the atmosphere. Ammonia volatilization occurs in soils with a pH of 7.0 or greater. Since many desert soils are basic, there is a large potential for ammonia volatilization from desert landscapes. Ammonia is formed by the loss of a proton from NH^ by the following reaction: NH4 + OH- => NH3 tl + H2O Ammonia is very soluble in water, therefore volatilization from moist soils is minimal. The loss of ammonia is greatest from coarse, dry soils with limited cation exchange capacity (Fleisher et a/., 1987). Ammonia volatilization measured in three habitats on a Chihuahuan Desert watershed demonstrated that wetting soils increased the rates of nitrogen mineralization and of ammonia volatilization (Schlesinger and Peterjohn, 1991). Another study found that irrigation decreased nitrogen mineralization which would have the effect of decreasing ammonia volatiHzation (Fisher et ai, 1987). The differences in results of these studies emphasizes the importance of long-term studies for understanding ecosystem processes in arid ecosystems. The studies by Schlesinger and Peterjohn were short-term incubations of soils enclosed in PVC pipe into which water and/or chemicals were added. The studies by Fisher et al. (1987) were conducted in the field over several years and incorporated 'droughted plots' under 'rain-out shelters', irrigated, and unmanipulated plots. The relationship between ammonia volatilization and nitrogen mineralization is affected by the effects of rainfall on development of the ephemeral plant community. In shrub-dominated systems, irrigation stimulates growth of dense stands of ephemerals. The decomposition of the dead roots of those plants results in nitrogen immobilization and reduction in


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mineralization rates with irrigation in the following year (Fisher and Whitford, 1995). Thus the relative importance of ammonia volatiUzation in desert ecosystems is dependent on climate patterns as well as spatial variability. Denitrification is another avenue of loss of nitrogen from the soil pool back to the atmospheric pool. Denitrification results from the microbial reduction of nitrate (NO3) or nitrite (NO2) to either NÔ or dinitrogen (N^). There are many species of heterotrophic soil bacteria that are capable of denitrification. Factors that have been shown to affect the rate of denitrification include: pH, temperature, available carbon, available nitrogen and the partial pressure of oxygen in the soil (Firestone, 1982). Desert soils are notoriously low in carbon and nitrogen, are frequently basic, and subject to extremes in temperature. Most desert upland soils are coarse textured and well aerated thus not good environments for denitrification. In most desert watersheds, losses of nutrients in litter and plant debris transported into ephemeral streams is a spatial phenomenon, i.e. concentration of organic materials at the watershed terminus. Nutrient losses from desert watersheds only occur in those few watersheds that are drained by perennial streams. Thus in most desert landscapes, denitrification is the most important means of loss of this nutrient. In a nitrogen budget developed for the Great Basin Desert in the western US, it was estimated that denitrification accounted for 95% of the annual nitrogen losses and 65% of the total N inputs (West and Skujins, 1977). Estimates of denitrification rates in the cold Great Basin Desert in North America (19 kg N ha~^ yr"^) were approximately equal to 65% of the nitrogen inputs. Other studies in the Great Basin estimated denitrification losses equal to approximately 80% of the nitrogen fixed by cryptogamic crusts and of added ^^N-labeled ammonium (Klubek and Skujins, 1981). In the Sonoran Desert, high rates of denitrification were measured in wet soil under nitrogen-fixing mesquite tree canopies (Virginia et a/., 1982). Denitrification was estimated to account for > 77% of the N inputs in the desert regions of the US (Peterjohn and Schlesinger, 1990). The content of denitrifying enzymes in upland desert soils was associated with indices of N and C availability. However, wet desert soils provided optimal conditions for the several variables that affect denitrification (Peterjohn and Schlesinger, 1991). In the Chihuahuan Desert denitrification rates were affected by water availability and the C/N ratio of the soil. Highest rates were measured in desert grassland and there were no 'fertile island' effects on denitrification rates in shrubland soils. Annual rates of denitrification on a Chihuahuan Desert watershed were estimated at 2.3 kg N ha~^ yr~^ on piedmont grassland soils, 4.3 kg N ha~^ yr~^ in creosotebush (L. tridentata) shrubland soils, 10.5 kg N ha~^ yr~^ in basin grassland soils, and 3.5 kg N ha~^ yr"^ in ephemeral lake soils. The differences in rates of denitrification in soils from various locations on the watershed were attributed to soil textural differences that affect soil water potentials (Peterjohn and Schlesinger, 1991). These studies suggest that denitrification is probably an important process in the ecosystems of most desert regions. However, caution must be applied when extrapolating the results of studies of denitrification, based on irrigated soil cores, to desert regions of the world. In incubations used to measure denitrification of

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soils, cores were flooded with the equivalent of a 31.5 mm rain (Peterjohn and Schlesinger, 1990). Rains of that intensity have a return time of less than one storm per 20 years in the area where these studies were conducted (Fisher and Whitford, 1995). Since low oxygen tensions are characteristic of supersaturated soils, measures of denitrification under such conditions clearly optimize denitrification. Although the spatial variation and ratio of soil carbon and nitrogen are important variables affecting desertification, rates of denitrification are probably considerably lower, with water inputs equivalent to an average rainfall event of 10-15 mm, than those reported in the literature based on saturated soil incubations. Most of the available nitrogen in the soil comes from the mineralization of organic nitrogen. Available soil nitrogen is affected by the relative rates of mineralization and immobilization. Immobilization of nitrogen occurs when ammonia or nitrate are incorporated into microbial biomass. Microbial biomass can change very rapidly under suitable conditions of substrate (available organic matter) and soil moisture. In desert ecosystems there appears to be an increase in microbial biomass, an increase in soil carbohydrates, and a decrease in extractable soil nitrogen in response to irrigation (Gallardo and Schlesinger, 1995). In desert grassland soils, fertilization with nitrogen increased microbial biomass but addition of carbohydrate increased microbial biomass only when accompanied by nitrogen fertilization. In shrub-dominated ecosystems, addition of carbohydrate resulted in increased microbial biomass and decreased extractable nitrogen and phosphorus (Gallardo and Schlesinger, 1995). Thus increases in microbial biomass requires different conditions in desert grasslands in comparison to desert shrublands. Mineralization of nitrogen involves the soil microfauna as well as the soil microflora. As pointed out earlier, much of the organic matter that serves as a source of nutrients for plant growth is in the form of high C:N ratio dead plant roots (especially roots of ephemeral plants). As soon as the roots of annual plants die, they are colonized by heterotrophic bacteria and fungi. In soils where available N is very low, the growth of microbes on the roots is limited by available N. Microbes scavange the available N and convert it into microbial biomass N. The N in microbial biomass is not available to plant roots until the microbial biomass dies and that microbial biomass N is released as inorganic N. When desert soils are wet (less than -0.3 MPa) there is a complex of soil microfauna that graze on the microbial biomass and release mineral N as excretory products. That soil microfauna is composed of protozoans, nematodes, and microarthropods. However, in dry soils, most of the soil microfauna is encysted or anhydrobiotic, leaving only a few taxa of soil mites to graze on the fungal hyphae (Whitford, 1989). During dry periods, some fungi grow slowly on the organic substrates in the soil and there is limited mineralization of N via the grazing activities of the few soil mites. The relative rates of N mineralization and N immobilization, therefore, are determined by the pulses of rainfall and pulses of inputs of organic substrates. Periodic drought results in the death of microbial biomass. Rainfall that ends such drought produces conditions for rapid mineralization of the dead


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microbial biomass and competition of the growing microbial biomass and plant roots for available nutrients (Fisher and Whitford, 1995). The differences in responses of shrubland and desert grassland to nitrogen fertilization appear to be related to the spatial and temporal distribution of ephemeral plants and to the growth characteristics of the dominant plants. In desert grasses the gradual turnover of root biomass and location of nitrogen-fixing organisms in the rhizosheaths of roots of some species produce conditions where nitrogen minerahzation rates always exceed rates of nitrogen immobilization (Fig. 9.11) (Whitford and Herrick, 1996). The temporal patterns of mineralization-immobilization on the dead roots of ephemeral plants also explains the temporal variation in productivity of annual plants in shrublands where that productivity is decoupled from rainfall (Guo and Brown, 1997). In the northern Chihuahuan Desert, high production of winter-spring annuals is linked to reductions in production of summer annuals even in years with above-average summer rainfall. The relationships between rainfall, plant productivity, and the relative rates of nitrogen mineralization to immobilization are complex. Rainfall that initiates growth and establishment of ephemeral plants sets up the potential for an input pulse of decomposable organic matter. Spatial distribution and densities of ephemerals are a function of the morphologies of the dominant vegetation. With C3 shrubs, a large residual pool of dead roots from the previous growing season

grassland shrubland with summer ephemerals only shrubland with ephemerals (summer + winter) growing season summer C4 grasses growing season winter - summer ephemerals

Figure 9.11 The hypothesized relationships between nitrogen mineralization and nitrogen mobilization in perennial desert grasslands and in desert shrublands. (From Whitford and Herrick, 1995.) With permission from The Society for Range Management.

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or a pool of dead roots from the winter-spring ephemerals, establishes conditions for nutrient immobilization and reduction in growth rates of the shrubs. In desert grassland dominated by C^ grasses, spatial distribution of ephemerals is random and pulses of dead roots do not result in rates of immobilization exceeding rates of mineralization (Whitford and Herrick, 1995) (Fig. 9.11). 9.6 OTHER POTENTIALLY LIMITING NUTRIENTS Phosphorus is frequently a limiting element in cultivated agricultural systems. In studies on a northern Chihuahuan desert watershed, there was no evidence for P hmitation of primary production (Lajtha and Schlesinger, 1988). However, phosphorus may be limiting in deserts with very ancient soils, e.g. Australian, South African arid regions, but there are no quantitative data available to support this supposition. There are also suggestions that other elements may be limiting factors in primary production in arid and semiarid ecosystems but there are no empirical studies that examine this suggestion. Thus for most materials the processes of decomposition and mineralization turn over sufficient quantities of elements for the water-limited plant growth. 9.7 ROLE OF MYCORRHIZAE Mycorrhizae (literally fungal roots) are fungi that develop mutualistic symbioses with roots of many species of plants. Mycorrhizal fungi are known to enhance the nutrition of the host plant (especially with respect to phosphorus nutrition) and receive benefit from the host plant in the form of useable energy: carbohydrates (Read et al., 1985). Mycorrhizae produce nondestructive interfaces with root cells as intercellular hyphae, intracellular coils, arbuscules and vesicals (Fig. 9.12). The external micorrhizal hyphae extend from the root surface into the soil where the hyphae serve as functional extensions of the roots. Some studies have documented mycorrhizal mycelial links between different species of plant (Watkins et al., 1996). Most of the research on mycorrhizal associations and the ecological significance of these symbioses has been conducted with agricultural plants or with naturally occurring host plants in mesic environments. Approximately 95% of the world's plant species belong to families that are typically mycorrhizal and most of these families form vesicular-arbuscular endomycorrhizae associations (Trappe, 1981). A survey of the literature on mycorrhizal colonization of roots of arid and semiarid plants revealed that some species in most of the plant families surveyed were colonized by mycorrhizae. Most of the reports were based on single observations but where multiple samples were examined, some samples were mycorrhizal and some were not, e.g. Atriplex canescens (Chenopodiaceae) (Trappe, 1981). Arid zone plants that were usually mycorrhizal were members of the Compositae, Gramineae, Leguminosae,


Figure 9.12

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Vesicular-arbuscular mycorrhizal structures in a root.

Rosaceae, Salicaceae, and Solonaceae. The Cruciferae and Zygophyllaceae were reported to be largely nonmycorrhizal. Out of 93 species of plants from 37 families in semiarid, open mallee {Eucalyptus spp.) 85 were mycorrhizal and most of the mycorrhizae were vesicular-arbuscular (McGee, 1986). Several genera that were not previously known to harbor mycorrhizae were found to be mycorrhizal. Some species of Cactaceae and Chenopodiaceae were mycorrhizal at some times and nonmycorrhizal at other times. Ephemeral species in families that are typically mycorrhizal appear to develop mycorrhizal symbionts as readily as the perennial species in those families. Vesicular-arbuscular mycorrhizae fruit as single spores in the soil. The concentrations of vesicular-arbuscular spores in the soil vary seasonally, with soil moisture, and probably with the phenological stage of the host plant. Mycorrhizal spores are dispersed by anything that moves soil (wind, water, and animals) (Allen et ai, 1989). A variety of arthropods (crickets, grasshoppers, ants, wasps, and other insects) and some small mammals have been found to be spore dispersers (Trappe, 1981). Mycophagous arthropods may adversely affect the symbiosis between vesicular-arbuscular mycorrhizae and plants by grazing on the spores and external hyphae. Some microarthropods may also act as symbiotic mutualists by grazing and disseminating fungal propagules or by stimulating fungal growth and activity by grazing (Moore, 1988). Harvester ants, Pogonomyrmex occidentalism leave high densities of vesicular-arbuscular mycorrhizal fungi in the soils of the nest mounds that assist in the estabhshment of mutualistic mychorrizal associations after the ant colonies have abandoned the mounds (Friese and Allen, 1993). The mutualism between mychorrizal fungi and plant roots is one in which the fungus receives carbon from the plant and the plant receives nutrients from the fungus. Mycorrhizal fungi differ in their ability to take-up and metabohze various

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carbon sources (Smith and Read, 1998). Vesicular-arbuscular mycorrhizae are totally dependent on photosynthetically derived glucose. The external mycelia of other mycorrhizae can use a greater variety of carbon sources. Root exudation may trigger mycorrhizal colonization but the quantities of exudate are apparently insufficient to support continuous mycorrhizal development. Mycorrhizal colonization may reduce and change the quality of root exudates (Ingham and Molina, 1991). There is some evidence that mycorrhizal plants are more drought tolerant than non-mycorrhizal species. This is thought to result from the increased soil volume exploited by mycorrhizal plants and not to an increase in uptake of water by the fungi (Ingham and Mohna, 1991). The enhancement of growth in mycorrhizal plants is generally attributed to the exploitation of a larger volume of soil by the mycorrhizal mycelia that are external to the root. Increased phosphorus and ammonium uptake and inflow to plant roots has been documented (Read et al., 1985). Direct connection of plant roots of different individuals of the same species or roots of different species by mycorrhizal bridges has also been documented (Robinson and Fitter, 1999). Also, there is evidence for the movement of nutrients from dying roots to living plants via mycorrhizal links between dying and living roots (Newman and Eason, 1989). Experimental evidence for the direct interplant transfer of carbon suggests that mycorrhizal associations may be extremely important for productivity of patches of plants with different rooting depths and phenologies. An immediate supply of carbohydrate could allow shallow-rooted plant species to initiate growth of photosynthetic tissue as soon as soil moisture becomes available. Mycorrhizal associations may not be the only root-fungus mutualism that improves the growth and survival of desert plants. The roots of dominant grass and shrub species of the arid southwestern United States, were extensively colonized by dark septate fungal endophytes (Barrow et aL, 1997). Dark septate fungi were either the only fungi in the root or were found in combination with vesicular-arbuscular mycorrhizae in the same root. The internal morphology of dark septate fungi is different from other mycorrhizal associations. The dark septate hyphae penetrate the root and grow inter- and intracellularly in the root cortex. Dark septate fungi form microsclerotia of various structural configurations over time. These microsclerotia take the form of coils and vesicles. Dark septate fungi are widespread and are abundant in stressed ecosystems suggesting that they are ecologically important (Jumpponen and Trappe, 1998). Dark septate fungal associations with the roots of a desert shrub, Atriplex canescens, were found to contain large quantities of lipid during the period of leaf production by the shrub in the early spring. It was hypothesized that the storage of carbon in the dark septate fungi during peak growth periods of the plant could serve as a carbon sink and enhance water retention in the fungal hyphae. From the fungal reservoir, sufficient carbon, water, and nutrients could be supplied to the plant to insure survival during periods of cHmatic stress (Barrow, 2001). This fascinating fungus-plant association should be the focus of future research on desert plants and nutrient cycling processes.


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REFERENCES Aerts, R. (1996). Nutrient resorption from senescing leaves of perennials: are there general patterns. J. EcoL 84, 597-608. Aerts, R. (1997). Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship. Oikos 79, 439-449. Allen, M. R, Hipps, L. E., and Wooldrige, G. L. (1989). Wind dispersal and subsequent estabhshment of VA mycorrhizal fungi across a successional arid landscape. Land. EcoL 2, 165-171. Barrow, J. R. (2001). The transfer of lipids to fourwing saltbush, Atriplex canescens (Pursh) Nutt. by a dark septate fungal root endophyte. /. Arid Environ. (In Press) Barrow, J. R., Havstad, K. M., and McCaslin, B. D. (1997). Fungal root endophytes in fourwing saltbush, Atriplex canescens, on arid rangelands of southwestern USA. Arid Soil Res. Rehabil. 11,177-185. Barth, R. C. and Klemmedson, J. O. (1978). Shrub-induced spatial patterns of dry matter, nitrogen and organic carbon. Soil Sci. Soc. Am. J. 42, 804-809. Belnap, J. and Harper, K. T. (1995). Influence of cryptobiotic soil crusts on elemental content of tissues of two desert seed plants. Arid Soil Res. Rehabil. 9, 107-115. Berg, B., Wickbohm, G., and McClaugherty, C. (1984). Lignin and holocellulose relations during the long-term decomposition of some forest litters. Long-term decomposition in a Scots pine forest. IV. Can. J. Bot. 62, 2540-2550. Cepeda-Pizaro, J. G. (1993) Litter decomposition in deserts: an overview with an example from coastal arid Chile. Rev. Chil. de Hist. Nat. 66, 323-336. Cepeda-Pizaro, J. G. and Whitford, W. G. (1990). Decomposition patterns of surface leaf Utter of six plant species along a Chihuahuan Desert watershed. Am. Midi. Nat. 123, 319-330. Charley, J. L. and Cowling, S. W. (1968). Changes in soil nutrient status resulting from overgrazing and their consequences in plant communities of semi-arid areas. Proc. Ecol. Soc. Aust. 3,23-38. Charley, J. L. and West, N. E. (1975). Plant-induced chemical patterns in some shrub-dominated semi-desert ecosystems of Utah. / Ecol. 63, 945-964. Charley, J. L. and West, N. E. (1977). Micro-patterns of nitrogen mineralization activity in soils of some shrub-dominated semi-desert ecosystems of Utah. Soil Biol. Biochem. 9, 357-365. Cox, G. W. and Gakahu, C. G. (1985). Mima mound microtopography and vegetation pattern in Kenyan savannas. /. Trop. Ecol. 1, 23-36. Cox, G. W and Gakahu, C. G. (1986). A latitudinal test of the fossorial rodent hypothesis of Mima mound origin. Z. Geomophol. 30, 485-501. Creusere, F. M. and Whitford, W. G. (1982). Temporal and spatial resource partitioning in a Chihuahuan Desert lizard community. In N. J. Scott Jr. (ed.), Herpetological Communities, pp. 121-127. U S Fish and Wildlife Service, Wildlife Research Report 13, U.S. Department of the Interior, Washington, DC. deSoyza, A. G., Whitford, W. G., Martinez-Meza, E., and Van Zee, J. W (1997). Variation in creosotebush (Larrea tridentata) canopy morphology in relation to habitat, soil fertility and associated annual plant communities. Am. Midi. Nat. 137, 13-26. Elkins, N. Z. and Whitford, W. G. (1982). The role of microarthropods and nematodes in decomposition on a semi-arid ecosystem. Oecologia 55, 303-310. Elkins, N. Z., Steinberger, Y., and Whitford, W G. (1982). Factors affecting the apphcability of the AET model for decomposition in arid environments. Ecology 63, 579-580. El Shahaby, A. F. (1988). Associative nitrogen fixation with C4 grasses of the northern Chihuahuan Desert. PhD. Dissertation. New Mexico State University, Las Cruces, NM. 134 pp. Ettershank, G., Ettershank, J., Bryant, M., and Whitford, W. G. (1978). Effects of nitrogen fertiUzation on primary production in a Chihuahuan Desert ecosystem. /. Arid Environ. 1, 135-139. Firestone, M. K. (1982) Biological denitrification. In F. J. Stevenson (ed.) Nitrogen in Agricultural Soils, pp. 289-326. Soil Science Society of America. Madison. Wisconsin. Fisher, F. M. and Whitford, W. G. (1995). Field simulation of wet and dry years in the Chihahuan Desert: soil moisture, N mineralization and ion-exchange resin bags. Biol. Eertil. Soils 20,137-146.

270


Fisher, E M., Parker, L. W., Anderson, J. P., and Whitford, W. G. (1987). Nitrogen mineralization in a desert soil: interacting effects of soil moisture and nitrogen fertilizer. Soil Sci. Soc. Am. J. 51, 1033-1041, Fisher, F. M., Zak, J. C , Cunningham, G. L., and Whitford, W. G. (1988). Water and nitrogen effects in growth allocation patterns of creosotebush in the northern Chihuahuan Desert. / Range Mgmt 41, 3S1-391. Fleisher, Z., Ravina, A. K. I., and Hagin, J. (1987). Model of ammonia volatilization from calcareous soils. Plant Soil 103, 205-212. Floret, C., Pontanier, R., and Rambal, S. (1982). Measurements and modelling of primary production and water use for a south Tunisian steppe. / Arid Environ. 5, 77-90. Friedel, M. H., Cellier, K. M., and Nicolson, K. P. (1980). Nutrient deficiencies in central Austrahan semi-desert rangelands, with reference to decline in range condition. Aust. Rangeland J. 2, 151-161. Friese, C. F. and Allen, M. F. (1993) The interaction of harvester ants and vesicular-arbuscular mycorrhizal fungi in a patchy semi-arid environment: the effects of mound structure on fungal dispersion and establishment. Funct. Ecol. 7, 13-20. Gallardo, A. and Schlesinger, W. H. (1995). Factors determining soil microbial biomass and nutrient immobihzation in desert soils. Biogeochemistry 28, 55-68. Garcia-Moya, E. and McKell, C. M. (1970). Contribution of shrubs to the nitrogen economy of a desert-wash plant community. Ecology 51, 81-88. Gehrke, C , Johanson, U., Callaghan, T. V., Chadwick, D., and Robinson, C. H. (1995). The impact of enhanced ultraviolet-B radiation on Utter quality and decomposition processes in Vaccinium leaves from the subarctic. Oikos 72, 213-222. Gile, L. H., Gibbens, R. P., and Lenz, J. M. (1998). Soil-induced variability in root systems of creosotebush (Larrea tridentata) and tarbush (Flourensia cernua). J. Arid Environ. 39, 57-78. Guo, Q. and Brown, J. H. (1997). Interactions between winter and summer annuals in the Chihuahuan Desert. Oecologia 111, 123-128. Gutierrez, J. R. and Whitford, W G. (1987). Chihuahuan Desert annuals: importance of water and nitrogen. Ecology 68, 2032-2045, Gutierrez, J. R., DaSilva, O. A., Pagani, M. I., Weems, D., and Whitford, W G. (1988). Effects of different patterns of supplemental water and nitrogen fertilization on productivity and composition of Chihuahuan Desert annual plants. Am. Midi. Nat. 119, 336-343. Hamadi, Z., Steinberger, Y., Kutiel, P., Lavee, H., and Bamess, G. (2000) Decomposition of Avena sterilis litter under arid conditions. J. Arid Environ. 46, 281-293. Hogberg, P. (1986). Nitrogen-fixation and nutrient relations in savanna woodland trees (Tanzania). J. Appl. Ecol. 23, 675-688. Ingham, E. R. and Molina, R. (1991). Interactions among mycorrhizal fungi, rhizosphere organisms, and plants. In P. Barbosa, V. A. Krischik, and C. G. Jones (eds). Microbial Mediation of Plant-Herbivore Interactions, pp. 169-197. John Wiley, New York. Jacobson, K. M. and Jacobson, P. J. (1998). Rainfall regulates decomposition of buried cellulose in the Namib Desert. J. Arid Environ. 38, 571-583. James, C. D. (1991). Temporal variation in diets and trophic partitioning by coexisting lizards (Ctenotus: Scincidae) in central Australia. Oecologia 85, 553-561. James, D. W. and Jurinak, J. J. (1978). Nitrogen fertilization of dominant plants in the northeastern Great Basin Desert. In N. E. West and J. Skujins (eds), Nitrogen in Desert Ecosystems, pp. 219-243. Dowden, Hutchinson and Ross, Stroudsburg, PA. Johnson, H. B. and Mayeux, H. S. Jr.(1988). Mesquite nodulation and nitrogen fixation on rangelands. Proceedings 41st Annual Meeting of Society for Range Management. Abstract 296. Johnson, H. B. and Mayeux, H. S. Jr (1990). Prosopis glandulosa and the nitrogen balance of rangelands: extent and occurrence of nodulation. Oecologia 84, 176-185. Jumpponen, A., and Trappe, J. M. (1998) Dark spetate endophytes: a review of facultative biotrophic root-colonizing fungi. New Phytol. 140, 295-310. Killingbeck, K. T. (1993) Nutrient resorption in desert shrubs. Rev. Chil. Hist. Nat. 66, 345-355.


271

Killingbeck, K. T. and Whitford, W. G. (2001). Nutrient resorption in shrubs growing by design and by default in Chihuahuan Desert arroyos. Oecologia (in press) Klemmedson, J. O. and Tiedeman, A. R. (1986). Long term effects of mesquite removal on soil characteristics: II. nutrient availability. Soil Sci. Soc. Am. J. 50, 476-480. Klubek, B. and Skujins, J. (1981). Gaseous nitrogen losses from ^^N-ammonium and plant material amended with labeled nitrogen in Great Basin Desert surface soils. Geomicrobiol J. 2,225-236. Lajtha, K. (1987). Nutrient resorption efficiency and the response to phosphorus fertilization in the desert shrub Larrea tridentata (DC.) Cov. Biogeochemistry 4, 265-276. Lajtha, K. and Schlesinger, W. H. (1988). The biogeochemistry of phosphorus cycling and phosphorus availability along a desert soil chronosequence. Ecology 69, 24-39. Lajtha, K. and Whitford, W. G. (1989). The effect of water and nitrogen amendments on photosynthesis, leaf demography, and resource-use efficiency in Larrea tridentata, a desert evergreen shrub. Oecologia 80, 341-348. Lee, K. E. and Wood, T. G. (1971). Termites and Soils. Academic Press, London. Lobry de Bruyn, L. A. and Conacher, A. J. (1990). The role of termites and ants in soil modification: a review. Aust. J. Soil Res. 28, 55-93. Loftis, S. G. and Kurtz, E. B. (1980). Field studies of inorganic nitrogen added to semiarid soils by rainfall and blue-green algae. Soil Sci. 129, 150-155. Loye-Pilot, M. D., Martin, J. M., and Morelli, J. (1990). Atmospheric input of inorganic nitrogen to the western Mediterranean. Biogeochemistry 9, 117-134. Ludwig, J. A. and Flavill, P. (1979). Productivity patterns of Larrea in the northern Chihuahuan Desert. In E. C. Lopez, T. J. Mabry, and S. F. Tavizon (eds), Larrea, pp. 139-150. Centro de Investigacion en Quimica Aphcada, Saltillo, Mexico. MacKay, W P , Silva, S., Lightfoot, D. C , Pagani, M. I., and Whitford, W G. (1986). Effect of increased soil moisture and reduced soil temperature on a desert soil arthropod community. Am. Midi. Nat. 116, 45-56. MacKay, W. P, Silva, S., Loring, S. J., and Whitford, W. G. (1987). The role of subterranean termites in the decomposition of above-ground creosotebush litter. Sociobiology 13, 235-239. MacKay, W. P., Zak, J., and Whitford, W G. (1992). Litter decomposition in a Chihuahuan Desert playa. Am. Midi. Nat. 128, 89-94. MacKay, W P , Loring, S. J., Zak, J. C , Silva, S. I., Fisher, R M., and Whitford, W G. (1994). Factors affecting loss in mass of creosotebush leaf-litter on the soil surface in the northern Chihuahuan Desert. Southwest. Nat. 39, 78-82. Mandimba, G., Heulin, T., Bally, R., Guckert, A., and Balandreau, J. (1986). Chemotaxis of freeliving nitrogen-fixing bacteria towards maize mucilage. Plant Soil 90,129-139. Mayland, H. F. and Mcintosh, T. H. (1966). Distribution of nitrogen fixed in desert algal-crusts. Soil Sci. Soc. Am. Proc. 30, 605-609. Mazzarino, M. J., Bertiller, M. B., Spain, C. L., Laos, F., and Coronato, F. R. (1996). Spatial patterns of nitrogen availability, mineralization, and immobilization in northern Patagonia, Argentina. Arid Soil Res. Rehabil. 10, 295-309. McClaugherty, C. A., Aber, J. D., and Melillo, J. M. (1984) Decomposition dynamics of fine roots in forested ecosystems. Oikos 42, 378-386. McGee, P. (1986). Mycorrhizal associations of plant species in a semiarid community. Aust. J. Bot. 34, 585-593. Meentemeyer, V. (1978). Macroclimate and lignin control of litter decomposition rates. Ecology 59, 465^72. Montana, C , Ezcurra, E., Carrillo, A., and Delhoume, J. P. (1988). The decomposition of litter in grasslands in northern Mexico: a comparison between arid and non-arid environments. J. Arid Environ. 14,55-60. Moore, J. C. (1988). The influence of microarthropods on symbiotic and non-symbiotic mutualism in detrital-based below-ground food webs. Agric. Ecosyst. Environ. 24,147-159. Moorhead, D. L. and Reynolds, J. F. (1989). Mechanisms of surface Utter mass loss in the northern Chihuahuan Desert: a reinterpretation. /. Arid Environ. 16, 157-163.

272


Moorhead, D. L., Fisher, R M., and Whitford, W. G. (1988). Cover of spring annuals on nitrogen-rich kangaroo rat mounds in a Chihuahuan Desert grassland. Am. Midi Nat. 20, 443-447. Mun, H. T. and Whitford, W. G. (1998). Changes in mass and chemistry of plant roots during longterm decomposition on a Chihuahuan Desert watershed. Biol. Fertil. Soils 26, 16-22. Nash, M. H. and Whitford, W. G. (1995). Subterranean termites: regulators of soil organic matter in the Chihuahuan Desert. Biol. Fertil. Soils 19, 15-18. Newman, E. I. and Eason, W. R. (1989). Cychng of nutrients from dying roots to living plants, including the role of mycorrhizas. In M. Clarholm and L. Bergstrom (eds), Ecology of Arable Land, pp. 133-137. Kluwer Academic Publishers, New York. Parker, L. W , Fowler, H., Ettershank, G., and Whitford, W G. (1982). The effects of subterranean termite removal on desert soil-nitrogen and ephemeral flora. /. Arid Environ. 5, 53-59. Parker, L. W, Miller, J., Steinberger, Y., and Whitford, W G. (1983). Soil respiration in a Chihuahuaa Desert rangeland. Soil Biology and Biochemistry 15, 303-309. Parker, L. W., Santos, P. R, Phillips, J., and Whitford, W G. (1984). Carbon and nitrogen dynamics during decomposition of litter and roots of a Chihuahuan Desert annual, Lepidium lasiocarpum. Ecol. Monogn 54, 339-360. Pauh, F. (1964). Soil fertility problem in arid and semi-arid lands. Nature 204, 1286-1288. Penning de Vries, F. W. T. and Djiteye, M. A. (1982). La Productivite des Paturages Saheliens. Centre for Agricultural Publishing and Documentation. Wageningen, Netherlands. Peterjohn, W. T. and Schlesinger, W. H. (1990). Nitrogen loss from deserts in the southwestern United States. Biogeochemistry 10, 67-79. Peterjohn, W T. and Schlesinger, W. H. (1991). Factors controlling denitrification in a Chihuahuan Desert ecosystem. Soil Sci. Soc. Am. J. 55, 1694-1701. Peterson, D. K. and Whitford, W. G. (1987). Foraging behavior of Uta stansburiana and Cnemidophorus tigris in two different habitats. Southwest. Nat. 32, 427-433. Pianka, E. R. (1985). Some intercontinental comparisons of desert lizards. Nat. Geogn Res. 1, 490-504. Read, D. J., Francis, R., and Finlay, R. D. (1985). Mycorrhizal myceha and nutrient cycling in plant communities. In D. Atkinson, D. J. Read, and M. B. Usher (eds). Ecological Interactions in Soil: Plants, Microbes, and Animals, pp. 193-217. Blackwell Scientific Publications, Oxford, England. Reddell, P., Bowen, G. D., and Robson, A. D. (1985). The effects of soil temperature on plant growth, nodulation, and nitrogen fixation in Casuarina cunninghamiana Miq. New Phytol. 101, 1-10. Reichle, D. E. (1971). Energy and nutrient metabolism of soil and litter invertebrates. In P. Duvigneaud (ed.). Productivity of Forest Ecosystems, pp. 465-477. UNESCO, Paris. Reinhold, B., Hurek, T., Niemann, E., and Fendrik, I. (1986). Close association of Azospirillum and diazotrophic rods within different root zones of Kollar grass. Appl. Environ. Microbiol. 52, 520-526. Robinson, D., and Fitter, A. (1999). The magnitude and control of carbon transfer between plants linked by a common mycorrhizal network. / Exp. Bot. 50, 9-13. Romney, E. M., Wallace, A., and Hunter, R. B. (1978). Plant responses to nitrogen fertihzation in the northern Mojave Desert and its relationship to water manipulation. In N. E. West and J. Skujins (eds). Nitrogen in Desert Ecosystems, pp. 232-243. Dowden, Hutchinson and Ross, Stroudsburg, PA. Rougier, M. and Chaboud, A. (1985). Mucilages secreted by roots and their biological functions. Isr J. Bot. 34, 129-136. Rundel, R W , Nilsen, E. T., Sharifi, M. R., Virginia, R. A., Jarrel, W M., Kohl, D. H., and Shearer, G. B. (1982). Seasonal dynamics of nitrogen cycling for a Prosopis woodland in the Sonoran Desert. Plant and Soil 67, 343-353. Santos, P. F. and Whitford, W G. (1981). The effects of microarthropods on litter decomposition in a Chihuahuan Desert ecosystem. Ecology 62, 654-663. Santos, P. F. and Whitford. W G. (1983). The influence of soil biota on decomposition of plant material in a gypsum sand dune habitat. Southwest. Nat. 28, 4 2 3 ^ 2 7 .


273

Santos, P. R, Phillips, J., and Whitford, W. G. (1981). The role of mites and nematodes in early stages of buried Utter decomposition in a desert. Ecology 62, 664-669. Santos, P. R, Elkins, N. Z., Steinberger, Y, and Whitford, W. G. (1984). A comparison of surface and buried Larrea tridentata leaf litter decomposition in North American hot deserts. Ecology 65, 278-284. Schaefer, D. A. and Whitford, W G. (1981). Nutrient cycling by subterranean termites in a Chihuahuan Desert ecosystem. Oecologia 65, 382-386. Schaefer, D., Steinberger, Y., and Whitford, W. G. (1985). The failure of nitrogen and hgnin control of decomposition in a North American desert. Oecologia 65, 382-386. Schlesinger, W. H. and Peterjohn, W. T. (1991). Processes controlling ammonia volatilization from Chihuahuan Desert soils. Soil Biol Biochem. 23, 637-642. Schlesinger, W H., Gray, J. T., and GiUiam, P. S. (1982). Atmospheric deposition processes and their importance as sources of nutrients in chaparral ecosystems of southern California. Water Resources Res. 18, 623-629. Schlesinger, W H., Raikes, J. A., Hartley, A. E., and Cross, A. R (1996). On the spatial pattern of soil nutrients in desert ecosystems. Ecology 77, 364-374. Seastedt, T. R. (1984). The role of microarthropods in decomposition and mineralization processes. Ann. Rev. Entomol. 29, 25-46. Shearer, G., Kohl, D. H., Virginia, R. A., Bryan, B. A., Skeeters, J. L., Nilsen, E. T., Sharifi, M. R., and Rundel, P. W. (1983). Estimates of N2-fixation from variation in the natural abundance of ^^N in Sonoran Desert ecosystems. Oecologia 56, 365-373. Shields, L. M. (1957). Algal and lichen floras in relation to nitrogen content of certain volcanic and arid range soils. Ecology 38, 661-663. Silva, S. I., MacKay, W P., and Whitford, W. G. (1985). The relative contributions of termites and microarthropods to fluff grass litter disappearance in the Chihuahuan Desert. Oecologia 67, 31-34. Smith, S. E. and Read, D. J. (1998). Mycorrhizal Symbiosis, 2nd edn. Academic Press, London. Steinberger, Y and Whitford, W G. (1983). The contribution of rodents to decomposition processes in a desert ecosystem. /. Arid Environ. 6, 177-181. Steinberger, Y and Whitford, W G. (1988). Decomposition process in Negev ecosystems. Oecologia 75, 61-66. Steinberger, Y, Shmida, A., and Whitford, W. G. (1990). Decomposition along a rainfall gradient in the Judean Desert, Israel. Oecologia 82, 322-324. Stephens, G. and Whitford, W G. (1993). Responses of Bouteloua eriopoda to irrigation and nitrogen fertilization in a Chihuahuan Desert grassland. /. Arid Environ. 24, 415-421. Strojan, C. L., Randall, D. C , and Turner, R B. (1987). Relationship of litter decomposition rates to rainfall in the Mojave desert. Ecology 68, 741-744. Swift, M. J., Heal, O. W., and Anderson, J. M. (1979). Decomposition in Terrestrial Ecosystems. Blackwell Scientific, Oxford . Thurow, T. L. (1989). Decomposition of grasses and forbs in coastal savanna of southern Somalia. Afr. J. Ecol. 27, 201-206. Trappe, J. M. (1981) Mycorrhizae and productivity of arid and semiarid rangelands. In Advances in Food Producing Systems for Arid and Semi-arid Lands, pp. 581-599. Academic Press, New York. Virginia, R. A. and Jarrell, W. M.. (1983). Soil properties in a mesquite-dominated Sonoran Desert ecosystem. Soil Sci. Soc. Am. J. 47, 138-144. Virginia, R. A., Jarrell, W. M., and Franco-Vizcaino, E. (1982). Direct measurement of denitrification in a Prosopis (mesquite) dominated Sonoran Desert ecosystem. Oecologia 53, 120-122. Virginia, R. A., Jenkins, M. B., and Jarrell, W. M. (1986). Depth of root symbiont occurrence in soil. Biol. Fertil. Soils 2, 273-284. Vossbrinck, C. R., Coleman, D. C , and Wooley, T A. (1979). Abiotic and biotic factors in htter decomposition in semiarid grassland. Ecology 60, 256-271. Wallwork, J. A. (1982). Desert Soil Fauna. Praeger, New York.

274


Watkins, N. K., Fitter, A. H., Graves, J. D., and Robinson, D. (1996). Carbon transfer between C3 and C4 plants linked by a common mycorrhizal network, quantified using stable carbon isotopes. Soil Biol Biochem. 28, 471-^77. Weltzin, J. F. and Coughenour, M. B. (1990). Savanna tree influence on understory vegetation and soil nutrients in northwestern Kenya. / Veget. Set 1, 325-334. West, N. E. and Skujins, J. (1977). The nitrogen cycle in North American cold-winter, semi-desert ecosystems. OecoL Plant. 12, 45-53. Whitford, W. G. (1989). Abiotic controls on the functional structure of soil food webs. Biol. Fertil. Soils 8, 1-6. Whitford, W G. (2000). Keystone arthropods as webmasters in desert ecosystems. In D. C. Coleman and P. F. Hendrix (eds), Invertebrates as Webmasters in Ecosystems, pp. 2 5 ^ 1 . CABI Pubhshing, New York. Whitford, W G. and DiMarco, R. (1995). Variability in soils and vegetation associated with harvester ant (Pogonomyrmex rugosus) nests on a Chihuahuan Desert watershed. Biol. Fertil. Soils 20, 169-173. Whitford, W G. and Herrick, J. E. (1995). Maintaining soil processes for plant productivity and community dynamics. In N. E. West (ed.), Proceedings of the Fifth International Rangeland Congress vol. II, pp. 33-37. Society for Range Management, Denver, CO. Whitford, W G. and Kay, F. R. (1999). Biopedturbation by mammals in deserts: a review. /. Arid Environ. 41, 203-230. Whitford, W G., Meentemeyer, V., Seastedt, T. R., Cromack, K. Jr, Crossley, D. A. Jr., Santos, R F., Todd, R. L., and Waide, J. B. (1981). Exceptions to the AET model: deserts and clear-cut forests. Ecology 62, 257-277. Whitford, W G., Repass, R., Parker, L. W , and Elkins, N. Z. (1982). Effects of initial litter accumulation and chmate on litter disappearance in a desert ecosystem. Am. Midi. Nat. 108,105-110. Whitford, W. G., Steinberger, Y., MacKay, W P, Parker, L. W , Freckman, D. R, Wallwork, J. A., and Weems, D. (1986) Rainfall and decomposition in the Chihuahuan Desert. Oecologia 68,512-515. Whitford, W. G., Reynolds, J. F., and Cunningham, G. L. (1987). How desertification affects nitrogen limitation of primary production of Chihuahuan Desert watersheds. In E. F. Aldon, C. E. Gonzales-Vincent and W. H. Moir (eds). Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones, pp. 143-153. General Technical Report RM 150. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO. Whitford, W G., Stinnett, K., and Anderson, J. (1988). Decomposition of roots in a Chihuahuan Desert ecosystem. Oecologia 75,147-155. Whitford, W. G., Ludwig, J. A., and Noble, J. C. (1992). The importance of subterranean termites in semi-arid ecosystems in south-eastern Australia. / Arid Environ. 22, 82-91. Whitford, W. G., Martinez-Turanzas, G., and Martinez-Meza, E. (1995). Persistence of desertified ecosystems: explanations and implications. Environ. Monit. Assess. 37, 319-332. Whitford, W.G., Anderson, J., and Rice, P. M. (1997). Stemflow contribution to the 'fertile island' effect in creosotebush, Larrea tridentata. J. Arid Environ. 35, 451-457. Wullstein, L. H., Bruening, M. L., and Bollen, W B. (1979) Nitrogen fixation associated with sand grain root sheaths (rhizosheaths) of certain xeric grasses. Plant Physiol. 46, 1-4.

Chapter 10

Desertification

Desertification has been described as the diminution or destruction of the biological potential of the land, and can lead ultimately to desert-like conditions. It is an aspect of the widespread deterioration of ecosystems under the combined pressure of adverse and fluctuating climate and excessive exploitation. Such pressure has diminished or destroyed the biological potential, i.e. plant and animal production, for multiple purposes at a time when increased productivity is needed to support growing populations in quest of development. (Verstraete, 1986).

The 1992 UN Conference on Environment and Development that was held in Rio de Janeiro, defined desertification as land degradation in arid, semiarid, and dry subhumid areas resulting from various factors including cHmatic variations and human activities. These definitions of desertification focus on degradation and economic decline in arid and semiarid regions of the world. The United Nations conferences focused attention on the human populations affected by desertification '250 million people around the world and an indirect threat to a further 750 milhon people' (Wilhams and Balling, 1996). The economic implications of desertification result primarily from a reduction in management options as a result of the degradation of land resources. The United Nations conference recognized that the immediate cause of degradation of the ecosystems was the interaction of variation in climate (drought) and overuse of the natural resources during these periods. When evaluating desertification, as with any widespread environmental degradation, it is necessary to separate the effects of disturbance, i.e. environmental factors that are within the range of the evolutionary history of the ecosystems, i.e. drought, flood, fire, and wind storms, from those effects that are anthropogenic and which impose stress on the ecosystems. The magnitude of the changes resulting from the combination of drought disturbance and stress by over-use of natural resources is the result of management decisions made by the land managers and indigenous people. Some aspects of desertification such as the harvesting of woody plants for fuel have adverse effects on the ecosystems that are independent of climatic disturbance. Desertification produces many changes in the ecosystems of a region. Desertification is recognized primarily by the physical changes in the environment: reduction of plant cover, soil loss, loss of soil organic matter, deposition of

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sand bodies, increased run-off, etc. (Kassas, 1977). In extreme cases, desertification can result in marked reduction in vegetative cover and loss of soil by water and wind erosion. In such areas there is a loss in primary productivity and in potential productivity. The impacts of desertification are not evenly distributed among the arid and semiarid regions of the world. Drought combined with high human population growth and poor land management practices had devastating effects on the humans in the Sahel and Ethiopia during periods in the 1970s and 1080s. More than 1.3 million people starved to death in this region from the combined effects of desertification and civil war. The result of the recurring droughts interacting with farming of marginal lands has resulted in alarming drops in agricultural production and loss of productive capacity in the northern margins of the Sahel (Darkoh, 1998). In most other parts of the world, desertification has adversely affected livestock production capacity and the economic status of the residents (Grove, 1986; Arshad and Rao, 1994; Omar and Abdal, 1994; Fullen and Mitchell, 1994; Dean et a/., 1995; Noble, 1997) but has not caused human death due to starvation. Not all desertified regions suffer a loss of primary productivity. In many regions, the loss of productivity is measured as reduction in the production of harvestable animals (livestock) which results from changes in species composition of the vegetation. In southern Africa recognition that desertification is occurring begins when desirable forage species are replaced by species that are largely inedible by livestock (Milton et al., 1994). In much of the southwestern US and in eastern Australia, desertification is recognized as the loss of forage species (primarily perennial grasses) and replacement of these species by woody shrubs that are largely inedible by livestock (Buffington and Herbel, 1965; Archer et al., 1988; Archer, 1990; Grover and Musick, 1990; Noble, 1997). This is well documented in the case study of the Jornada Experimental Range in southern New Mexico (Buffington and Herbel, 1965). The percentage of land area classified as grassland (with few or no shrubs) decreased from 60% in 1858 to 0% by 1970. In 1858, the land area dominated by mesquite, Prosopis glandulosa, accounted for 20% of the land area and this increased to 60% by 1970. At the present time, more than half of the land area dominated by mesquite is in the form of coppice dunes. Coppice dunes form when blowing sand accumulates around the stems of multistemmed mesquite plants. The plants continue to grow, producing adventitious roots into the accreting dune. Mature coppice dunes may exceed 4 m in height and be stabilized by a coppice of 20-50 cm stems that form a crown of more than 4 m in diameter. The conversion of grassland to coppice dunes reduces the livestock forage to nearly zero and effectively eliminates livestock production as a use of that land area. In the Australian arid and semiarid zones, desertification is referred to as the woody weed problem. In Australia the problem is primarily one of loss of palatable species and replacement with nonpalatable largely woody species (Noble, 1997). In the semiarid region at the southern edge of the Atacama, desertification resulted from the loss of perennial grasses and the increased cover of nonpalatable

Desertification

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small tree, Acacia caven (FabaceaeiMimosoideae). The resulting landscape is a pseudo-savanna consisting of the trees and a large variety of native and exotic annuals (Ovalle et al., 1993). Similar changes have occurred as a result of degradation in the semiarid and arid regions of Argentina, i.e. invasion of the small tree, Geojfroea decorticans (Fabaceae).

10.1 HISTORY OF DESERTIFICATION Desertification is a problem with ancient roots. Cutting of forests, overgrazing and salt accumulation in irrigated lands led to desertification in Mesopotamia and the lands bordering the Mediterranean more than 2000 years ago. There is evidence that the climate of the Mediterranean region has been in a drying phase for much of the past 5000 years (El Baz, 1983). However, in the Mediterranean region, the climatic drying was coincident with developing agricultural technology and the increased demand for resources such as timber. The development of city-states was dependent upon the grain fields, timber, and animal products from the semiarid lands fringing the Mediterranean. As these resources were overexploited, the processes of desertification such as soil loss, reduced the productivity of the region. The declining productivity resulting from desertification contributed to the collapse of major civilizations such as Carthage, Greece, and the Roman Empire. The long history of degradation of the lands surrounding the Mediterranean had parallels in India and southwestern Asia and western China associated with technological advances in the cultures of those regions. However, it was not until the 20th century and the industrial revolution that technology allowed development of groundwater and opened up vast areas of the arid and semiarid lands of the world to commercial livestock production and to irrigation of lands not adjacent to rivers. The technological advances of the 20th century also allowed exponential growth of human populations. The rapidly expanding human populations increased the pressure on the natural resources of the arid and semiarid regions. This has resulted in the spread of desertification to every continent except Antarctica. In some areas such as the Sahel, drought combined with intensification of resource use has had devastating effects on the ecosystems and the human inhabitants. Desertification is a continuing process in many areas of the world. The productivity of semiarid lands around the world continues to be reduced by the increase in unpalatable woody plants. This coupled with reduction in vegetative cover leads to further deterioration of soil structure and soil hydraulic properties. Now more than 75% of the area that can potentially be desertified has already been affected to some degree by desertification processes (Karrar and Stiles, 1984). The history of desertification in the 20th century may best be illustrated by a case study of the southwestern US. By examining the progress of desertification in a region with a well-documented history, we may identify some of the underlying causes of degradation and gain some insights into the degradation process.

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Bahre and Shelton (1993) have documented the vegetation changes that have occurred in southeastern Arizona since 1870. These include: decline in grass, increases of woody xerophytes, decline in riparian wetlands, decline in desert scrub and cactus communities, and expansion of exotics. Most but not all of these changes have occurred in New Mexico and west Texas since 1870 (Hastings and Turner, 1965; Grover and Musick, 1990). Not only has vegetation changed but there have been less well-documented changes in the soils (erosion, deposition) and drainages (change from perennial flow to ephemeral flooding, stream bed cutting, and head cutting). The degradation of western rangelands was well recognized by the turn of the 20th century (Griffiths, 1901). Griffiths (1901) described decreased rangeland productivity because of the destruction of grasslands by overstocking. For example 'There were fully 50,000 head of stock at the head of Sulphur Spring Valley and the valley of the Aravipa in 1890. In 1900 there was no more than onehalf that number and they were doing poorly' (Griffiths, 1901). Changes in vegetation were accompanied by soil erosion, arroyo cutting and changes in soil structure (Gibbens et al., 1983; Grover and Musick, 1990). Although there is ample evidence that desertification has occurred and is continuing on these desert rangelands there is no agreement among scientists and land managers about the causes of these changes. Overgrazing and other land-use practices are frequently cited as primary desertification by some groups while climatic drought and competition between native grasses and shrubs are invoked as causal agents by other groups (Bahre and Shelton, 1993). However, responses to questionnaires sent by Griffiths (1901) to ranchers in southeastern Arizona consistently attributed the degradation to overstocking. Degradation of southeastern Arizona grasslands was documented in a letter from C. H. Bayless of Oracle, Arizona to Griffiths: . . . the San Pedro Valley consisted of a narrow strip of subirrigated and very fertile lands. Beaver dams checked the flow of water and prevented cutting of a channel. Trappers exterminated the beavers and less grass on the hillsides permitted greater erosion, so that within four to five years a channel varying in depth from 3 to 20 feet (approximately 1 m to 6 m) was cut almost the whole length of the river. . . . Of the rich grama grasses that originally covered the country, so little now remains that no account can be taken of them.. . . Where stock water is far removed, some remnant of perennial grasses can be found. Grasses that grow only from seed sprouted by summer rains are of small and transitory value. The foliage of mesquite and catclaw bushes is eaten by most animals and even the various cacti are attempted by starving cattle. . . . No better pasture was ever found in any country than that furnished by our native grama grasses, now almost extinct.. . . The present unproductive conditions are due entirely to overstocking.. . . Twelve years ago, 40,000 cattle grew fat along a certain portion of the San Pedro where now 3,000 cannot find sufficient forage for proper growth and development. If instead of 40,000 head, 10,000 had been kept on this range, it would in all probability be furnishing good pasture for the same number today. Very few of these cattle were sold or removed from the range. They were simply left there until the pasture was destroyed and the stock then perished by starvation.

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Griffiths (1901) included letters from a number of other ranchers that basically told the same story. The degradation of rangelands in southeastern Arizona was not confined to two decades before the beginning of the 20th century. Recent studies in the San Pedro drainage show that desert grassland fragments have continued to diminish in size and become more isolated as small islands surrounded by a sea of shrubland (Kepner et al, 1999). Their analysis showed that the number of grassland patches increased by 61% and the average patch size decreased by 60% between 1974 and 1987. This fragmentation involved the loss of grass cover, replacement of palatable grasses with unpalatable shrubs, and soil loss due to the decrease in vegetation cover. The increase in areas dominated by woody species is frequently called brush encroachment because most of the shrub-tree species have been components of the vegetation for thousands of years. The driving force for the increase in cover and density of woody shrubs in desert grasslands 'seems to be chronic, high levels of herbivory by domestic animals' (Van Auken, 2000). The history of rapid degradation of arid rangelands in the southwestern US is paralleled by the history of degradation of eastern Australian rangelands. Degradation in the eastern Australian rangelands in the 1890s was exacerbated by the 'rabbit problem' which led to accelerated erosion of topsoil. It took only one season to destroy the plant cover and replace it with 'great patches of scalded clay, as bare as on the day when the last wavelet of some receding ocean lapped over them and left them to evolve a covering for their nakedness' (Noble, 1997). In 1900 a Royal Commission was established to examine the conditions of the rangelands in the Western Division of New South Wales. That commission listed the causes of the degradation of the rangelands that led to the economic depression in the region: low rainfall, rabbits, overstocking, sandstorms, and growth of nonedible scrub and fall in prices. According to the pastoralist who testified at the Royal Commission hearings the system of stocking which has been practised during the last twenty years, with the result that the majority is eaten out, leaving only isolated specimens to propagate the species upon the return of more favoured conditions . . . at present is producing vegetation capable of adapting itself to its altered surroundings by its unpalatableness and protective spiny growths . . . is apparent to every casual observer who notes the entire absence of grasses or herbage amongst the dense growth of injurious scrub on the red lands, which fifteen years ago were open well-grassed country (Noble, 1997).

The changes in vegetation and soils in the eastern Australian rangelands resulted from the interaction between the natural variability in rainfall and overuse of the palatable vegetation during periods when low rainfall limited productivity. Many of the perennial herbaceous and grass species were unable to cope with the grazing stress imposed during climatic conditions that placed the plants at their tolerance limits. Change in species composition and the spatial distribution of vegetative cover plus soil loss are not the only impacts of historical episodes of desertification.

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Dramatic changes in the structure of landscapes can occur in a short period of time. One of the major consequences of the loss of grass cover on the watersheds in southeastern Arizona was the shift from low-energy overland flow of run-off water through broad grass covered swales, to high-energy, flash-flood, overland flow. Historical sources indicate that the gullying and arroyo head cutting that characterizes the present-day landscapes of southeastern Arizona occurred in the period 1865-1915 with a peak in the 1880s (Cooke and Reeves, 1976). Today the San Simon and San Pedro valleys are deeply dissected landscapes with steepsided ephemeral channels draining the sparsely vegetated uplands (Fig. 10.1). There is evidence that the changes described as desertification occur even in the absence of grazing. There was no increase in perennial grass cover in a grazing exclosure established in 1933 and on an area partially occupied by mesquite, P. glandulosa, when re-measured in 1980. There were no differences between the grazing exclosure and grazed area transects that were grassland in 1935 and that had degraded to mesquite dunes when re-measured in 1980 (Hennessy et aL, 1983). Similar results were obtained in grazing exclosure studies in Arizona where the densities of mesquite plants doubled between 1932 and 1949 in both cattle-excluded and grazed areas (Glendening, 1952). During that same period, there was a 20-fold decrease in densities of perennial bunch grasses. In a comparison of grazed and ungrazed paddocks estabhshed in 1941, Smith and Schmutz (1975) recorded some recovery of grasses and reduction in subshrubs in the

1^ igure 10.1 The deeply incised channel of the San Simon River in southeastern Arizona. The sparse creosotebush shrubland of the uplands replaced the desert grassland in the 1880s.

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ungrazed paddock but a large increase in mesquite was recorded in both paddocks. These authors also reported that an exotic, Lehmann lovegrass {Eragrostis lehmanniana) was establishing on both sites thereby contributing to the differences between paddocks. Vegetation change and degradation may continue in areas where grazing is excluded (Gibbens and Beck, 1988). Animal impact is not absolutely essential for ecosystem deterioration to continue. The results of these and other studies suggest that once shrub establishment and grass cover reduction has passed some threshold, the degradation trajectory is followed even when the environmental stress (grazing) is removed. The presence of some shrubs provides a seed source, which can then be distributed over varying distances by animals such as rodents. Seed dispersal distances and relative success of plant estabhshment are dependent on the size and behavior of the seed feeder. Rodents have been suggested as important dispersers of shrub seeds into grasslands even in the absence of grazing (Cox et al., 1993). The problem with the interpretation of these studies is that the areas excluded from grazing are rarely much larger than 2.3 km^ and most are considerably smaller. Although rodents may be capable of effectively dispersing seeds in small exclosures, it is less likely that they can account for shrub dispersal into areas that are hundreds of square kilometers in size. Another factor affecting continued change in exclosures even in the absence of grazing is that the surrounding landscape continues to be impacted by grazing which can lead to the development of erosion cells (Pickup, 1985; Pickup and Chewings, 1986). The growth of erosion cells eventually envelop exclosures, destroying grasses by burial in wind-blown sand, abrasion, and by the loss of topsoil by sheetflow from up-slope disturbed areas. The end result is that either with or without continued grazing, the areas change sufficiently to be identified as desertified. The most important historical development in the desertification story has been the development of technology that allows exploitation of deep, groundwater supplies. Deep wells, windmills and pumps have made all potential grazing land available to livestock. The development of pastoral industries in the semiarid regions of the world in the 20th century has centered on establishing reliable supplies of water for livestock. Established water points result in concentric areas of decreasing degradation radiating out from the water point reflecting the concentric nature of animal activity around water (Andrew and Lange, 1986a). Within 2.5 years after the establishment of a well and trough in a near pristine chenopod shrubland in South Australia, there were measurable changes due to the activity of sheep. At this location sheep were stocked at the lowest levels in the Austrahan arid zone. Soil changes included marked reduction in surface lichen crusts, increased soil compaction and increased bulk density, especially within 40 m of the trough. Vegetation changes included reduction in biomass of Atriplex vesicaria, a palatable shrub, and short-lived perennial grasses. Changes also included establishment of an exotic species, Marsubium vulgare, hore hound, within a 13 m radius from the trough. There was substantial mortality of A. vesicaria close to the trough after 8 years of grazing. Andrew and Lange (1986a) emphasize that

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these changes occurred despite the low stocking rate in the paddock. In addition, water points tend to concentrate the activities of native herbivores thereby exacerbating the grazing and browsing effects. Andrew and Lange (1968b) cite an extreme case where 6000 sheep denuded an area of saltbush up to 100 m from water in the first 3 months of stocking. Reductions of vegetative cover in the disturbance gradients around water points produce the initial erosion cells, which expand by abrasion and burial of vegetation plus soil loss by erosion. Human activities contributing to desertification are not limited to overgrazing or overutilization of plants that are palatable for livestock, harvesting trees, shrubs and dung for fuel decreases productivity of the land, opens areas to wind and water erosion and contributes to the loss of soil fertility (Fernandez and Busso, 1999). 10.2 ECOSYSTEM AND LANDSCAPE CONSEQUENCES OF DESERTIFICATION Most attention focused on the process of desertification of arid and semiarid lands has been on reduction in cover and loss of desirable plant species, invasion by woody plants, and soil erosion. The conceptual framework that has served as the working model for scientists and managers attempting to deal with desertification has been an autogenic succession model. Encroachment by shrubs has frequently been viewed as 'invasion' by species that were superior competitors with the more desirable plant species. Experience with grazing exclosures not only caused questioning of overgrazing as a causal agent of desertification but also caused us to question the applicability of autogenic succession models. If the ecosystem does not recover or even move in a trajectory toward the prestressed state when the environmental stressor is removed, is an autogenic succession model applicable? For semiarid rangelands, a conceptual model of desertification must allow for ecosystems to change structurally and functionally to alternate stable states, to experience discontinuous and irreversible transitions, and/or to establish nonequilibrium communities. Westoby et al. (1989) proposed a nonequilibrium conceptual model for rangelands which allows for rangelands to transition to one or more states that are not in equilibrium. The resulting rangeland communities may assume more than one configurafion (Fig. 10.2) depending on the interaction of a number of physical and biological variables. Rangeland ecosystem dynamics may best be described by catastrophe theory (Lockwood and Lockwood, 1993). Rangelands exhibit five characteristics of catastrophe systems: (1) distinct conditions or states of existence; (2) conditions that are very unstable; (3) relatively rapid movement between states; (4) hysteresis, i.e. processes associated with degradation or recover are not readily reversible by simply inverting the sequence of events; and (5) relatively small changes in initial conditions can result in dramatically different outcomes with time. A nonequilibrium, state and transition model is consistent with the structural changes in

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Pre-Anthropogenic Disturbance Transition Ecosystem State

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HE2 Figure 10.2 A conceptualization of the dynamics of arid and semiarid ecosystems in response to environmental stressors. Transition state may be either similar to or dissimilar to the eventual stable state, which is generally structurally and functionally different from the prestressed ecosystem.

desert rangelands that have been recorded in many areas of the world but there are systems in which the succession conceptual model seems to apply. Although the catastrophe model captures both the continuous (succession) and discontinuous (state and transition) dynamics, it is most applicable to the discontinuous dynamics that characterize most arid and semiarid ecosystems that are inhabited by humans. Change from one stable state to an alternate stable state requires a conceptual model for desertification that accounts for the resulting changes in spatial and temporal patterns of limiting resources. The limiting resources are water and nutrients. The environmental stressors that initiate the trajectory of change is the interaction between overgrazing by domestic livestock and rainfall amounts that are close to the threshold for survival of the dominant vegetation. Large numbers of domestic livestock not only reduce the cover of palatable species, soil-trampling effects are severe especially in the vicinity of water points. Reduced vigor of perennial grasses and herbaceous plants exacerbates the effects of trampling by livestock. Even short duration, high-intensity grazing by cattle can result in significant changes in the microtopography of landscape units. Reduction in

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vegetation cover increases the susceptibility to surface erosion by either wind or water. Reduction in cover of grasses and herbaceous perennials produces scattered patches of unvegetated soil where shrub seedlings can establish with no competition between the seedling roots and the roots of perennial grasses and herbaceous plants. Evidence for the importance of bare patches in heavily grazed grassland as establishment sites for shrubs was provided by a study where seedling creosotebushes {Larrea tridentata) were transplanted into nitrogen fertilized-irrigated, irrigated, and unmodified plots in an ungrazed black-grama (Bouteloua eriopoda) grassland, a creosotebush shrubland, and a grazed grassland with a mixture of grasses. After two years, the only site where cresotebush seedlings survived was in the grassland that had been intensively grazed prior to the initiation of the study (Whitford et aL, 2001). A conceptual model for desertification in the Chihuahuan Desert proposed that the reduction in vegetative cover resulting from excess grazing pressure plus drought resulted in a redistribution of soil resources. In desert grasslands, the distribution of soil nutrients is relatively uniform as is the distribution of soil water following rain events (Schlesinger et aL, 1990). Loss of grass cover produced open patches that were subject to overland flow of run-off on sloping terrain, and/or movement of materials by wind irrespective of slope. Wind and water redistributed organic materials such as rabbit dung, grass fragments and leaves, and surface soil particles. The loss of grasses eliminated efficient transfer of water from the leaves via stemflow to the root mat thus reducing infiltration rates. Erosion of the soil surface also eliminates biopores, which further reduces infiltration rates. Thus reduction in grass cover quickly resulted in the spatial redistribution of the critical water and nutrient resources. Unvegetated patches within such a landscape provided germination-establishment sites for shrubs. In the northern Chihuahuan Desert, the 'increaser' shrubs are creosotebush and mesquite (P. glandulosa). The distribution patterns of these species prior to the development of a commercial livestock industry was creosotebushes on steep, well-drained, gravelly or sandy slopes and mesquite along drainage channels, edges of ephemeral lakes and Indian encampments (York and Dick-Peddie, 1969). Episodes of mesquite and creosotebush invasion and establishment frequently followed reduction in grass cover resulting from drought and grazing (Grover and Musick, 1990). Creosotebush seed dispersal is primarily by overland water flow. Mesquite seed dispersal is primarily by animals that feed on the seedpods but do not destroy the seeds. Seedling mesquite are frequently seen emerging from cattle dung pats during summer rains in the Chihuahuan Desert. Seedlings of both mesquite and creosotebush rapidly produce deep taproots and long lateral roots near the surface of the soil (Hennessy et aL, 1983) (Figs 10.3 and 10.4). Replacement of grasses by shrubs set the stage for concentration of litter under shrubs and within coppices of shrubs thereby creating 'islands of fertility' or 'resource islands' (Fig. 10.5). Organic fragments are trapped in a very different way in shrub-dominated ecosystems than they are in grassland ecosystems. The

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Figure 10.3 A remnant patch of black grama {Bouteloua eriopoda) desert grassland on the Jornada Experimental Range in southern New Mexico.

Figure 10.4 Mesquite {Prosopis glanduosa) coppice dunes that replaced black grama {Bouteloua eriopoda) desert grassland on the Jornada Experimental Range in Southern New Mexico.

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Ecology of Desert Systems ORIGINAL BLACK GRAMA GRASSLAND Uniform distribution of resources Disturbance (grazing, drouglit)

1

Redistribution of Resources (Water and nutrients)

INCREASING SHRUBBINESS

Z

Q Gain of Resources j Aeolian Dune Slirub Resource islands Run-off/Run-on

X

(^ Loss of Resources^ >Run-off >Soil Erosion

Figure 10.5 A diagram of the Jornada conceptual model for desertification. Redistribution of water and nutrients involves both spatial and temporal changes in availability of these resources.

architecture of the shrubs determines their characteristics with respect to creating surface roughness and eddy currents in wind and with respect to their efficacy as obstructions to sheet flow of water (DeSoyza et al., 1997a). Mesquite (R glandulosd), which grew as a single-stemmed plant in most areas prior to the introduction of large herds of livestock, assumed a multiple-stem morphology in the degrading grassland This multistemmed mesquite trapped blowing sand thus forming the nucleus for a coppice dune. As the stems were buried by blowing sand, buried stems developed adventitious roots and the meristematic stem tips formed the coppice. Although the Jornada conceptual model is useful and is obviously apphcable to desertification processes in the Chihuahuan Desert, it does not fit all cases of desertification. There are examples of desertification in which the system moves from a heterogeneous spatial structure to a more-homogeneous spatial structure and as a consequence, temporal variability may be reduced. For example, in regions where the vegetation is structured into a mosaic of patches of bunch grasses and shrubs, loss of bunch grasses may actually increase the homogeneity of the desertified ecosystems. The common operational feature in the process is a change in the spatial and temporal heterogeneity of the ecosystems not simply an increase in heterogeneity. The change in ecosystem characteristics both temporally

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and spatially should lead to an increase in the variabihty of ecosystem processes and rates of processes. The processes involved in the spread of mesquite in the desert grasslands of North America differ in several ways from shrub invasion in the South African Karoo. In the Karoo, Acacia karoo, is the most important woody invader of grassland. Germination of A. karoo did not occur in livestock dung nor did reduction of grass cover enhance the establishment of this shrub (O'Conner, 1995). However, many of the consequences of the establishment of A. karoo in grasslands of southern Africa are similar to those experienced in North America with the spread of mesquite. Desertification in the Australian semiarid and arid regions also parallels the changes in ecosystem structure and function reported for North American rangelands. The impacts of domestic livestock combined with limited rainfall contribute to changes in the spatial heterogeneity of rangeland landscape units. In the Australian rangelands fine-scale heterogeneity is replaced by large-scale heterogeneity associated with the changes in abundance and spatial distribution of woody species (Tongway and Ludwig, 1990; Ludwig and Tongway, 1995). Changes in vegetation species composition and the spatial patterns of distribution of those species modify run-off patterns and the abundance and distribution of unvegetated patches that are subject to wind erosion. The conceptual model for degradation of Australian rangelands is very similar to the Jornada model. The ecosystem changes resulting from the spread of invasive shrubs in the South American arid zone also fit the Jornada model. Invasive shrubs, such as Acacia caven, result in the redistribution of soil and water resources. The changes in spatial and temporal availability of these critical resources result in positive feedback loops that exacerbate the degradation of the rangelands (Aronson et ai, 1993). Degradation of semiarid and arid rangelands in shrubs and trees that are unpalatable to livestock replace much of the grass and herbaceous cover produces changes in the spatial and temporal distribution of water and nutrients. When the modified spatial and temporal patterns of water and nutrient distribution have been in place for a sufficient period of time, one or more of the 'thresholds of irreversibility' have been crossed. The configuration of the degraded ecosystem will remain in the 'degraded' condition unless massive intervention is imposed (Aronson et ai, 1993). Degradation of arid and semiarid ecosystems does not always involve the replacement of desirable forage species by less desirable species. Degradation can result from the spread of nonnative species and the environmental consequences of abundant exotics. The Great Basin Desert in North America has been colonized by a Eurasian annual grass, Bromus tectorum, colloquially known as cheat grass along with tumbleweeds, Salsola spp., and a succulent halophyte, Halogeton glomeratus. In years with adequate precipitation, the intershrub spaces in the sagebrush-dominated rangelands develop a nearly complete cover of alien annual grasses and forbs. Although cover and abundance of alien species are greater in heavily grazed areas than in ungrazed or lightly grazed areas, they appear to be

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increasing in all Great Basin shrubland ecosystems. These alien species increase fuel loads sufficiently to greatly shorten the fire return cycles. Following fires, recolonization of sites by chenopods and sagebrush may be compromised by changes in disease organisms in the soil and inhibition of establishment by the presence of the alien species (Harper et al, 1996). Intense fire in these shrublands can leave the hillsides devoid of vegetation cover and susceptible to wind and water erosion. Fuel loads by alien species can result in fire in more arid regions where sparse vegetation could not carry fire. For example, red brome (Bromus rubens) has spread rapidly in the Mojave Desert and the upper Sonoran Desert of southern Nevada and Arizona. Wet winters since 1976 have encouraged the spread ofB. rubens and the buildup of fine fuels in the native shrublands. Large fires have been frequent in desert communities that rarely burned in the past. Desert plants such as saguaros (Camegia gigantea) that grow slowly and recruit episodically are essentially eliminated by these fires (Betancourt, 1996). Another alien grass that is rapidly expanding in the Sonoran Desert is Cenchrus ciliare. C. ciliare is providing fine fuel in areas of the Sonoran Desert where fire was absent or extremely rare. The longterm effects of expansion of alien annual grasses in desert rangelands can only be surmised because this is a very recent phenomenon. The increasing frequency of fire will undoubtedly change the spatial and temporal patterns of distribution of water and nutrients. These changes will undoubtedly affect the structure and processes of the ecosystems that develop in desert areas exposed to frequent fire. Desertification effects on temporal and spatial distribution of soil resources produces variability that changes coefficients of variation in biomass production and vegetation cover. For example, temporal changes in the chemical profiles (both quanUtafive and qualitative) of shrub-dominated ecosystems results in increased variability in foliage as a source of food for herbivores. The Jornada conceptual model focused on the physical processes that redistributed soil and nutrients but did not address any of the implications of changes in the composition of the animal communities nor changes in the spatial and temporal redistribution of animals in degrading grasslands. The establishment of commercial livestock production was accompanied by a number of activities designed to 'improve' the desert grasslands for livestock production. These activities included extirpation of prairie dogs (Cynomys ludovicianus), trapping and poisoning rodents such as the banner-tail kangaroo rats (Dipodomys spectabilis), and trapping, shooting, and poisoning predators such as the lobo wolf (Canis lupus), mountain lion (Felis concolor), and coyote {Canis latrans). The poisoning campaigns resulted in killing large numbers of small predators such as grey fox (Urocyon cinereoargenatus), skunks (Mephitis spp.) and badger (Taxidea taxus). The reduction in small predators probably led to an increase in the abundance of desert rodents that was noted by early workers at research stations such as the Jornada Experimental Range (unpublished reports, 1912-1918) despite the rodent control efforts. Recent studies have documented the effects of rodents on productivity, species composition, and landscape patchiness (Moorhead et ai.

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1988; Brown and Heske, 1990; Mun and Whitford, 1990; Chew and Whitford, 1992; Guo, 1996; Kerley and Whitford, 2000). The complex relationships between ecosystem function and desert rodents have led to a consideration of these animals as keystone species (Brown and Heske, 1990). Since the cessation of rodent control efforts, the rodent populations have reached high levels. Population densities of rodents are highest in the creosotebush shmblands on the Jornada and lowest in the desert grasslands. There are feedbacks between rodent abundance and impacts on the ecosystem. In ecosystems where bunch grasses provide as much as 10% of the vegetative cover, some species of rodents destroy a large fraction of the flowering tillers thereby affecting seed production and the seed bank (Kerley et al, 1997). Because there are no quantitative data on the relative abundance and species composition of the desert grassland rodent communities prior to the onset of the desertification processes, we have no baseline against which to assess the effects of changes in desert rodent communities that were concomitant with changes in vegetation and soils. However, there are large differences in abundance and species composition of rodent communities in remnant patches of desert grassland and rodent communities in shrublands (Whitford, 1993). Because rodents affect soil processes by their burrow systems, seed-caching behavior, and by granivory and gramnivory, they contribute a positive feedback in the maintenance of shrub communities once these are estabUshed (Whitford, 1993). In North American desert grasslands, the extirpation of the black-tail prairie dog (Cynomys ludovicianus) may have contributed to the spread of shrubs into the grassland and contributed to the extinction of other desert grassland species (Miller et a/., 1994). The consequences of desertification are not limited to biological impacts and reduction of productivity. Desertification affects a number of biophysical variables: albedo, temperature, precipitation, soil moisture, wind erosion, and water erosion. In semiarid regions where vegetation cover protects soil from wind erosion, degradation processes that reduce plant cover and increase fetch lengths of unvegetated patches result in an increase in wind erosion. Dust generated from erosion of recently exposed surfaces contributes to the global deposition of dust over the oceans and in land areas not subject to wind erosion (Arimoto et a/., 1989; Derbyshire et al, 1998). 10.3 RESISTANCE AND RESILIENCE OF DESERTIFIED LANDSCAPES Evidence is accumulating from experimental studies that, once established, shrubdominated ecosystems are both resistant to disturbance and resihent following perturbation. Field experiments using 'rain-out' shelters to impose summer drought on plots in creosotebush shrubland showed that the annual growth increments of plants subjected to drought were not significantly different from the growth of controls. Summer drought was imposed on the plots for five consecutive years.

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There were no differences in growth increments in thefifthyear of the experiment. Leaffitterfafi patterns were affected by the drought but the annual quantity of leaf litter production was the same for shrubs subjected to drought and control shrubs (Whitford et ai, 1995). There were similar responses to imposed drought in mesquite shrubs. Small mesquite shrubs exhibited greater decfine in photosynthesis when subjected to drought than the large shrubs. The results of this study suggested that large, estabfished plants were better able to withstand drought than were the small, establishing shrubs (DeSoyza et al, 1997b). However, after five consecutive years of growing-season drought, perennial herbaceous plants and perennial grasses disappeared on the drought plots (Whitford et al, 1995). In creosotebush, growth was suppressed during the growing-season drought. The shrubs exposed to drought exhibited rapid growth after the first rainfall after 'rain-out' canopies were removed. This growth spurt compensated for the lack of growth during the growing season. The rapid growth spurt was attributed to the higher rates of nitrogen mineralization in the soils under 'rain-out' shelters. The resilience of creosotebush to long-term, repetitive drought is attributed to its ability to utilize deep soil moisture and the soil system's nitrogen mineralization responses to drought (Whitford et al, 1995). Fifteen years after creosotebush seedlings were planted in a grazed desert grassland community, there was essentially no other perennial vegetation under the canopies of the shrubs and very low cover of annual plants. The soils under the recently established creosotebushes had lower aggregate stability than soils in adjacent plots where creosotebush had not established (Whitford Qt ai, 2001). These studies suggest that a number of mechanisms account for the resistance and resilience of shrubs that establish in degrading rangelands. Shrubs modify the water infiltration patterns by stemflow, throughfall fractions. Stemflow water may be channeled to deep soil layers along root surfaces. The shallow roots of shrubs probably compete with the root systems of perennial grasses and herbaceous plants for water and nutrients. Many shrubs are resistant to repetitive drought whereas grasses and herbaceous plants are not. In addition, the capacity of shrubs to respond rapidly to rainfall allows them to take advantage of the water and soil nutrient availability more rapidly than grasses and herbaceous plants. This provides resilience to disturbance in shrub dominated ecosystems. 10.4 DESERTIFICATION AND CLIMATE CHANGE Human activities that alter the ocean-atmosphere interactions, that change atmospheric and stratospheric chemistry and that alter land surface - atmosphere interactions directly affect regional cHmates around the world. Although it is uncertain how global warming will affect the thermal and chemical properties of the oceans, it is clear that changes in oceanic surface temperatures will impact the weather patterns of arid and semiarid regions. The linkages between ocean surface temperatures and precipitation patterns in the world's arid regions have been

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described in general terms but there is insufficient information to allow robust predictions of the effects of global warming on the climates of the worlds arid regions (Williams and Balling, 1996). Overgrazing that reduces vegetation cover initiates biogeophysical feedbacks that modify the climate of an arid or semiarid region. Reduced plant cover results in increased albedo. Increased albedo decreases the net radiation from the soil surface. Decreased net radiation from the soil surface results in surface cooUng. Surface cooling promotes subsidence of upper atmosphere air masses. Subsidence decreases convection and cloud formation which leads to reduced precipitation (Chamey, 1975; Chamey et al, 1975, 1977). Humans have affected the earth surface-atmospheric processes primarily by reducing vegetation cover by overgrazing, cultivation, and deforestation. Changes in vegetation cover alters surface roughness, thereby modifying wind speeds. Changes in vegetation cover also alter soil moisture patterns. Reduction in vegetation cover exposes soils to erosion and increases atmospheric dust loads. Atmospheric dust affects surface temperatures and provides raindrop nuclei. Burning of fuel woods and of savannas contributes to atmospheric particulates and trace gasses that impact the climate of arid and semiarid regions. There are no quantitative studies of these effects; therefore the net effect on the climate arid regions cannot be evaluated nor can generalizations be made about the impact of these on future climate change (WilUams and Balling, 1996). Global circulation models provide predictions of global and regional-scale climate changes. The model simulations are generally considered to be uncertain at regional geographic scales. The models predict increases in annual average temperature of 4-5°C for most of the worlds desert regions. For North American deserts, precipitation and soil moisture changes are 'highly variable by season and model' (Williams and BalUng, 1996). Predictions for precipitation in South American deserts are variable but 'summer-time soil moisture levels are generally expected to decrease' (WiUiams and BaUing, 1996). Lower precipitation is predicted for deserts in the Middle East and northeastern Africa and southern Africa. Increased winter rainfall is expected in the Asian arid lands with decreased soil moisture in summer. Increased rainfall in both summer and winter is predicted for the central Australian deserts. There are also predictions of increased frequency of drought in most of these regions. Global climate changes are likely to increase climate variability of the arid and semiarid regions of the planet. Whatever the direction and magnitude of climate changes are, the end result is that these changes will simply exacerbate desertification processes resulting from human activities in the future.

10.5 SOCIAL AND ECONOMIC CONSEQUENCES OF DESERTIFICATON The most severe social and economic impacts of desertification are seen in sub-Saharan Africa, the Sahel. Beginning with the severe drought in the 1970s

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and continuing today, overgrazing, fuelwood harvesting, and cultivation of marginal lands have drastically reduced per-capita food production in the region and increased the severity of poverty in an already impoverished area. Increasing poverty, food shortage, problems with land ownership and land tenure patterns interact with the rapid increase in human population to create social upheaval. Desertification forces large numbers of people to abandon subsistence agriculture and to emigrate to urban areas in hopes of finding employment and economic stability. In many areas of the Sahel, this migration is primarily of young adult males which leaves an inadequate labor force to sustain the subsistence agriculture (Darkoh, 1998). In some of the countries in the region government policies encourage permanent immigration into the semiarid and arid lands from urban areas. The immigration of culturally nonadapted peoples into these regions further exacerbates desertification. These migrants settle into these fragile environments with inappropriate technologies and no knowledge of sustainable management practices. The intrusions of row-crop agriculture into traditional pastoral lands creates civil strife in many parts of this region. In much of the region there have been repeated episodes of breakdown of law and order in the last few decades. These breakdowns have frequently resulted in mass exodus of people who are held in refugee camps in neighboring countries (Darkoh, 1998). Other aspects of the economies of the countries of the region lead to increasing the severity of desertification. A prime example is the demand for charcoal in the urban areas. The value of charcoal makes cutting small trees attractive to entrepreneurs. In most of the semiarid and arid lands in Africa, the economic costs of soil erosion resulting from desertification have been estimated by a World Bank study. Average US dollar losses by soil erosion were estimated at $80 to $150 per hectare for grazing lands. In much of Africa desertification 'translates into a spiral of declining production, increasing poverty and diminished potential productivity. It exacerbates poverty, which, in turn, exacerbates desertification because, as the pressure increases, people are forced to exploit their land to survive.. . . Entire societies and cultures are now threatened. The pastoralists of the Sahel are a case in point. For most, the loss of their livelihood means a life in relief camps or shanty towns mushrooming around Sahelian cities and those of the countries to the south' (Darkoh, 1998). Although the social and economic costs of desertification are less severe in Eurasia than in Africa, the loss of production potential and increasing human populations produce a positive feedback system that impacts the human communities especially in Pakistan, western India, Kazakstan, Turkmenistan, Uzbekistan, and Afganistan. There are social and economic consequences of desertification even in countries with robust economies. In the western US, the social and economic consequences of continuing degradation of rangelands are the cause of intense conflicts among the various user groups of the public rangelands. Environmental organizations claim that ranching is destroying the production base and adversely affecting plant and animal diversity and abundance on the

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rangelands. Ranchers and pastoralists are trying to protect a 'life-style' and see themselves as stewards of the land and its resources. The economic realities are frequently overlooked by both groups. For example, ranchers in the desert grassland region of the southwestern United States realize a return on investment of only 1-3%. 'From an investment standpoint, Chihuahuan Desert cattle ranching would be considered unprofitable and risky by any Wall Street analyst' (Holachek, 1992). Changes in wholesale cattle prices, government policies such as drought subsidies, and climate fluctuations combine to influence management decisions by ranchers that frequently result in further degradation. Ranches on rangeland that is considered in fair to poor condition are frequently 'hobby ranches' where the desirable 'cowboy lifestyle' is subsidized by employment in another profession (Holachek, 1992). Ranches that are primarily poor to fair condition rangeland have to be managed on the basis of short-term financial returns rather than on sustaining the long-term health of the resource. Economic necessity (repayment of bank loans, feed a family, maintain fences, water points and other infrastructure etc.) forces ranchers to maximize herd size. This leads to conflicts with government land management agencies and environmental groups. When the conflicts cannot be resolved by compromise, they are moved to the court system where the resolution is rarely satisfactory to any of the litigants.

10.6 EFFECTS OF DESERTIFICATION ON BIODIVERSITY One of the major biological challenges of the 21st century is the resolution of the relationship between the diversity of organisms in ecosystems and the long-term behavior of those ecosystems. There are four prevalent hypotheses concerning this relationship: the diversity-stability hypothesis, the rivet hypothesis, the redundancy hypothesis, and the idiosyncratic response hypothesis (Johnson et al., 1996). The diversity-stability hypothesis predicts that ecosystems will increase in productivity and the ability to recover from disturbance as the number of species in the system increases. According to this hypothesis, deletion of species from the complex trophic webs will result in an increase in susceptibility of the system to disruption in response to a perturbation or environmental stress. The rivet hypothesis views species in ecosystems as the rivets holding an airplane together. The removal of rivets beyond some threshold number may cause the plane or ecosystem to crash. Implicit in this hypothesis is the view that a few extinctions may have little effect on ecosystem stability and resilience but as extinctions increase, the redundant components are lost and the ecosystem fails to resist disturbance and fails to rebound after experiencing disturbance. The redundancy hypothesis views ecosystem structure as species associations in functional groups. Extinction of a redundant species in a functional group will result in a neighbor species assuming the 'work' of the extinct species in the ecosystem. The idiosyncratic hypothesis views the relationship as intermediate between species composition

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and ecosystem function because of higher-order interactions among the biotic communities. Species richness in deserts is moderate to high in semiarid regions (> 200 mm rainfall per year) and declines with increasing aridity for most groups of organisms (Huenneke and Noble, 1996). Species richness appears not to be a good measure of biodiversity with respect to ecosystem function in arid and semiarid regions because species richness is frequently higher in the 'degraded' ecosystems than in the relatively unchanged systems. There is some evidence for 'keystone' species or species complexes that have multiple direct and indirect effects on ecosystem processes and structure. Loss of a 'keystone' species may compromise the ability of the system to resist environmental stress and the ability to recover structure and function after changes resulting from stress. The importance of 'keystone' species is not considered in species richness or species diversity measures. Loss of keystone species may be the critical feature of desertification that results in the ecosystem remaining in an altered stable state. Empirical data suggest that changes resulting from desertification have little effect or at least little negative effect on species richness. The alternate, stable configuration of shrubland ecosystems that replaced grassland systems, supported higher species richness and diversity of small mammals and birds in the desertified systems than in the relatively undisturbed desert grasslands (Whitford, 1997). However, in desertified systems dominated by an alien species (an introduced South African grass, Eragrostis lehmanniana) species richness of birds was significantly lower than in desertified systems with no alien species. In the systems dominated by E. lehmanniana, there were fewer taxa of 'ecosystem engineers' that affect soil processes and significantly less soil turnover than in systems without the ahen grass (Kulas, 2001). Taxa or functional groups that are most important in maintaining the integrity of arid ecosystems are those species that directly or indirectly affect soil processes (Whitford, 1996). The species richness of the soil biota in arid ecosystems appears to be more related to fine temporal and spatial scale patterns of organic materials than to the dominant vegetation. Soil compaction may affect the composition of the soil biotic community by reducing pore neck size in the pedon but soil compaction is generally a feature of intense impact of large herbivores in the immediate vicinity of a water point. Subterranean termites are a keystone component of many arid ecosystems. Termites appear to be very resistant to ecosystem degradation and structural change. There was no change in the spatial and temporal abundance and feeding rates of the 'keystone' subterranean termites in desertified and intact Chihuahuan Desert ecosystems (Nash et al, 1999). The importance of termites in nutrient cycling and energy flow is maintained even in ecosystems in which vegetation and soils have experienced dramatic change within less than a century. As desertification proceeds, relatively intact 'pre-anthropogenic stress' ecosystems are reduced in size and become fragments or 'islands' in the sea of changed

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landscape. This fragmentation frequently leads to local extinction of species, particularly top predators. For example a desert grassland specialist, the Aplomodo falcon (Falcofemoralis), has disappeared from the Chihuahuan Desert grasslands in the United States and survives as a sparse population in Mexico. 10.7 REHABILITATION OF DESERTIFIED LANDSCAPES Efforts to rehabilitate desertified landscapes have been focused on recovery of some if not all of the production potential of the rangelands. In some desertified regions, rehabilitation has focused on stabilizing mobile sands in order to protect transportation routes and human settlements from being buried under sand dunes. In desert rangelands where small trees and shrubs have invaded grasslands, rehabilitation efforts have focused on reducing shrub cover and re-establishing grasses. In North America, efforts to rehabilitate degraded rangelands have focused on control of shrubs. Herbicide applications, root plowing, chaining, and burning have been widely used in desertified areas of western US rangelands. Chaining involves pulling a large chain strung between two large tractors across a landscape where trees and shrubs are uprooted by the chain. Root plowing is accomplished by a heavy steel plow that is more than 1 m in length pulled through a brushcovered area by a large tractor. Root plowing and front-end stacking used to reduce brush density had no major long-term effect on brush recovery. Grass seeding following brush removal by root plowing and stacking also had no long-term effect on brush reinfestation (Gonzalez, 1990). The rapid reinfestation of cleared areas by shrubs and the absence of any long-term effect of mechanical shrub removal have resulted in the virtual abandonment of this as a rehabilitation technique. Herbicides are generally applied aerially but some research on hand application has been conducted (Figs 10.6 and 10.7). Fire is a management tool that is limited to areas and time periods when there is sufficient vegetation cover and biomass to carry a fire. In many areas where brush cover is a problem, there are insufficient fuel loads to carry fire. Frequently the brush control measures are combined with seeding. Seed mixtures are frequently native and alien species of grass but in many instances seeding by an alien grass species has been the preferred treatment (Call and Roundy, 1991). Rehabilitation of degraded rangelands has been attempted on extensive areas. Rehabilitation efforts are expensive and the benefits are not accrued for a sufficient period of time to recoup the expenditure. For example, between 1940 and 1981, an average of 600,000 h of brush-infested rangeland in Texas was treated annually by herbicides, fire, and mechanical means such as root-plowing, chaining, and disking (Rappole et ai, 1986). Despite the intensity of treatments, land managers realized only a transitory, short-term gain in productivity. Regrowth of shrubs occurred in less than a decade, and retreatment became necessary within 15 years of root plowing, 2 years after chaining and 8 years after treatment with

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Figure 10.6 A mesquite {Prosopis glandulosa) coppice dune with an ephemeral herb, Descurainia pinnata, following treatment with a herbicide that killed the mesquite shrubs, Jornada Experimental Range, southern New Mexico.

Figure 10.7 Mesquite coppice dunes 10 years after treatment with a herbicide, Jornada Experimental Range, southern New Mexico.

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herbicides (Rappole et aL, 1986). Similar results have been reported in other desert rangeland areas (Herbel et ai, 1983). Even when combined with exclusion of livestock grazing, root-plowing and herbicide treatment has not resulted in reestablishment of grassland (Chew, 1982; Roundy and Jordan, 1988). Partial success in restoring some of the functions of desert grassland has been achieved by seeding nonnative grasses. These grasses have revegetated watersheds and reduced damage from flash floods. They also provide forage for livestock at certain times of the year and on some microsites. For example, after initial screening by the US Department of Agriculture, Lehmann's lovegrass (Eragrostis lehmanniand) was seeded on selected sites in southeastern Arizona and southern New Mexico. Between 1940 and 1980, land managers in Arizona established Lehmann's lovegrass on 70,000 ha. Lehmann's lovegrass has since spread to an additional 130,000 ha (Cox and Ruyle, 1986; Anable tt aL, 1992). In New Mexico where Lehmann's lovegrass was seeded along electric transmission line corridors and road verges, it has spread into some adjacent landscape units. Where Lehmann's lovegrass has spread into native black-grama (Bouteloua eriopoda) and drop-seed (Sporobolus spp.) grasslands, it appears to be displacing the native grasses. Despite the benefits of having this exotic grass provide soil cover and forage for livestock in shrub dominated ecosystems, the diversity of fauna remains severely reduced (Bock et al, 1986). The spread of this exotic grass has stirred debate about the costs and benefits of restoring some of the functions of a desert grassland (Figs 10.8 and 10.9).

Figure 10.8 An example of restored desert grassland dominated by the alien Lehmann's lovegrass, Eragrostis lehmanniana, on the Santa Rita Experimental Range, southern Arizona.


Figure 10.9 A remnant patch of native desert grassland on the Santa Rita Experimental Range, southern Arizona.

Rehabilitation of desertified rangelands in Australia has focused on the same techniques that have been applied in North America. Blade plowing, chemical treatment and chaining were too energy-expensive to apply to large areas and were therefore not cost effective (Noble, 1997). Contour furrowing in patterned chenopod shrubland in an attempt to reduce erosion and encourage the establishment of desirable vegetation actually exacerbated the degradation (MacDonald and Melville, 1999). Because of the dispersive nature of the soils, the furrow banks were breached by the accumulation of sheet flow. The breached furrows created 'stream flow' which increased erosion. Differential leaching of cations in the furrow soils appears to be the cause of establishment of unpalatable (to livestock) plant species in the furrows. This example demonstrates the need for careful evaluation of soils, vegetation, and landscape-scale processes when designing rehabilitation projects as was emphasized by the authors. 'Care must be taken when utilizing this landscape to avoid further degradation through erosion, through changes in cation distribution, and through associated vegetation changes' (MacDonald and Melville, 1999). Mechanical rehabilitation methods have been widely applied on degraded rangelands in South Africa. The degraded condition occurring on each farm or nature reserve can be regarded as a unique unit with regard to soil, climate, and degree of degradation'(Van der Merwe and Kellner, 1999). In a study of the relative success of mechanical methods (moldboard plow vs. multiple-tined rippers)

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the deep-furrow plow achieved the best results on 'shrink-swell' clay soils and the multiple tined ripper afforded the best results on sandy soils. The mechanical modification of the landscape changed infiltration patterns, provided seed traps, and sites for organic matter concentration. The success of rehabilitation in the South African rangelands also depended on deferment of grazing for between 1 and 3 years to allow establishment and growth of vegetation. Grazing pressure and the effects of trampling are determinants of the longevity and effectiveness of the hollows and furrows (Griffith et al, 1984). Combinations of burning and application of chemical herbicides have proven to be the most cost effective in the short-term. Strategies combining mechanical and biological treatments have proven to be effective in some instances. Goats were used to browse shrubs that were regenerating after mechanical treatment. Shrub control by goat browsing following bulldozing was significantly better than either goats or bulldozing alone (Noble, 1997). In China a number of soil stabilization and rehabilitation techniques are used (Fullen and Mitchell, 1994). These include planting windbreaks, establishment of straw or clay checkerboards on bare dune surfaces and planting of xerophytes. In order to provide an environment where indigenous xerophytic plants can colonize and survive, it is essential to stabilize the surface material. In areas where mobile sand dunes are a problem, the Chinese have devised a strategy for establishing an artificial ecosystem on mobile dunes. The first stage in this strategy is the installation of a 'sand barrier' made from woven willow branches or bamboo to act as a windbreak. Behind the sandbarrier, straw checkerboards are established. Between 4.5 and 6 metric tons per hectare of wheat or rice straw are embedded in the sand in a grid of 1-3 m squares. The straw is inserted 15-20 cm deep so that the straw protrudes approximately 10-15 cm above the dune surface. Where available, clay is used instead of straw because of its greater durability. The checkerboards remain intact for 4-5 years. This allows time for the cultivated xerophytic plants that are placed within the squares to become established. A number of native shrubs and grasses are planted in the checkerboards. Stability of the surface is enhanced by the colonization and establishment of microbiotic crusts. These biocrusts also protect the surface from wind unless the crust is damaged by trampling or by burrowing animals. These rehabilitation methods have changed mobile dunes to stable productive ecosystems. Twenty-five years after establishment of artificial vegetation cover, more than ten additional species have colonized the area and vertebrates have increased from two to more than 30 species (Fuller and Mitchell, 1994). Although these are impressive results, the methods are labor intensive and may be prohibitively expensive for most countries where mobile sand dunes threaten human communities and/or infrastructure. Effective rehabilitation of desert rangelands in Tunisia was dependent upon the degree of degradation. In areas where there were sparse populations of native plant species, eliminating grazing pressure by exclosing livestock, resulted in

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reestablishment of many native plant species and partial stabilization of the aeolian landscape (Aronson et ah, 1993). In the most degraded sites, a selection of species that were known to be productive and resistant to livestock grazing were replanted on a small scale. However, for replanting to be successful, wind breaks had to be installed to stabilize the sand dunes and provide stable germinationestablishment sites. In addition, successful selection of species for rehabilitation was based on a large body of experimental data that had been accumulated on native species thought to be keystone species in these ecosystems (Aronson et ai, 1993). In Chile, the savanna (espinal) dominated by Acacia caven and alien annuals with its low soil fertility and low livestock productivity occupies the region that was a dense mosaic of forest and shrublands prior to occupation by European settlers (Ovalle et al., 1999). In this region an aggressive intervention was deemed necessary to improve the ecological and economic condition of the degraded espinales. The degraded soils exhibited precipitous declines in organic matter, water storage, duration of water storage and available nutrients. The degraded soils require the introduction of nitrogen-fixing legumes for revegetation and amending soils with appropriate microbial inocula (Aronson et aL, 1993). The appropriate microbial inocula include Rhizobium spp. and a 'cocktail' of vesicular-arbuscular mycorrhizae. Inoculation with both groups of symbionts appears to be important because Rhizobium requires mycorrhizal association to provide the high amounts of phosphorus needed for nodulation and nitrogen fixation (Ovalle et ai, 1999). In ten years of rehabilitation efforts based on planting nitrogen-fixing legumes and improvement of soil fertility, there has been improvement in productivity and soil fertility. Current efforts focus on re-establishment of native tree and herbaceous plants in an effort to facilitate the spontaneous re-establishment of native plants, microbes, and animals (Ovalle et aL, 1999). This case study demonstrates the need for defining achievable goals for a rehabilitation program and the necessity for management flexibility during the program. In most of the arid regions of the earth, a large fraction of the land area has been desertified or is somewhere on the trajectory towards desertification. The vast spatial extent of the areas requiring rehabilitation (millions of hectares) and the high costs of large-scale efforts demonstrate the need for low-cost, small spatial scale approaches to improving the condition of degraded landscapes. Successful rehabilitation requires identification of those parts of a landscape where low-intensity intervention will result in improved soil fertility, establishment of desirable plant species and improve habitat for native animals. In areas where degradation has not proceeded to an undesirable end-point, modification of land and livestock management may be all that is necessary to halt and reverse the process. In the landscapes where removing the stress (such as excluding domestic livestock) does not result in any meaningful recovery, active intervention is required. Rehabilitation of such areas requires a careful survey of current landscape and patch-scale processes and identification of those parts of the landscape

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where inexpensive intervention will provide the most effective and long-lasting improvements in landscape function. REFERENCES Anable, M. E., McClaran, M. P., and Ruyle, G. B. (1992). Spread of introduced Lehmann lovegrass, Eragrostis lehamanniana Nees. in southern Arizona, USA. Biol. Conserv. 61, 181-188. Andrew, M. H. and Lange, R. T. (1986a). Development of a new piosphere in arid chenopod shrubland grazed by sheep. 1. Changes to the surface. Aust. J. EcoL 11, 395-409. Andrew, M. H. and Lange, R. T. (1986b). Development of a new piosphere in arid chenopod shrubland grazed by sheep. 2. Changes to the vegetation. Aust. J. Ecol. 11, 411-424 Archer, S. (1990). Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas, U.S.A. /. Biogeogr. 17, 453-462. Archer, S., Scifres, C , and Bassham, C. R. (1988). Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland. Ecol Monogr. 58, 111-127. Arimoto, R., Duce, R. A., and Ray, B. R. (1989). Concentrations, sources and air-sea exchange of trace elements in the atmosphere over the Pacific Ocean. In J. P. Riley, R. Chester, and R. A. Duce (eds). Chemical Oceanography, pp. 107-149. Academic Press, San Diego. Aronson, J., Floret, C , Le Floc'h, E., Ovalle, C , and Pontanier, R. (1993). Restoration and rehabilitation of degraded ecosystems in arid and semi-arid lands. II. Case studies in southern Tunisia, central Chile and northern Cameroon. Restoration Ecol. 1, 168-187. Arshad, M. and Rao, A. R. (1994). The ecological destruction of Chohstan Desert and its ecoregeneration. UN Environ. Programme Desertification Control Bull. 24, 32-34. Bahre, C. J. and Shelton, M. L. (1993). Historic vegetation change, mesquite increases, and climate in southeastern Arizona. /. Biogeogr 20, 489-504. Betancourt, J. L. (1996). Long- and short-term climate influences on southwestern shrublands. In J. R. Barrow, E. D. McArthur, R. E. Sosebee, and R. J. Tausch (eds). Proceedings: Shrubland Ecosystem Dynamics in a Changing Environment, pp. 5-9. General Technical Report INT-GTR338. US Department of Agriculture, Forest Service Intermountain Research Station, Ogden, UT. Bock, C. E., Bock, J. H., Jepson, K. L., and Ortega, J. C. (1986). Ecological effects of planting African lovegrasses in Arizona. National Geogr Res. 2, 456-463. Brady, W. W., Stromberg, M. R., Aldon, E. R, Bonham, C. D., and Henry, S. H. (1989). Response of a semidesert grassland to 16 years of rest from grazing. J. Range Mgmt 41, 284-288. Brown, J. H. and Heske, E. J. (1990). Control of a desert grassland transition by a keystone rodent guild. Science 250, 1705-1707. Buffmgton, L. C. and Herbel, C. H. (1965). Vegetational changes on a semidesert grassland range from 1858 to 1963. Ecol. Monogr 35, 139-164. Call, C. A. and Roundy, B. A. (1991). Perspectives and processes in revegetation of arid and semiarid rangelands. J. Range Mgmt 44, 543-549. Charney, J. G. (1975). Dynamics of desert and drought in the Sahel. Q. J. R. Meteorol. Soc. 101, 193-202. Charney, J. G., Stone, P. H., and Quirk, W. J. (1975). Drought in the Sahara: a biogeophysical feedback mechanism. Science 187, 434-435. Charney, J. G., Quirk, W. J., Chow, S., and Kornfield, J. (1977). A comparative study of the effects of albedo change on drought in semi-arid regions. /. Atmos. Sci. 34, 1366-1385. Chew, R. M. (1982). Changes in herbaceous and suffrutescent perennials in grazed and ungrazed desertified grassland in southeastern Arizona, 1958-1978. Am. Midi. Nat. 108,161-169. Chew, R. M. and Whitford, W. G. (1992). A long-term positive effect of kangaroo rats (Dipodomys spectabilis) on creosotebushes (Larrea tridentata). J. Arid Environ. 22, 375-386. Cooke, R. U. and Reeves, R. W. (1916) Arroyos and Environmental Change in the American Southwest. Clarendon Press, Oxford.

302


Cox, J. R. and Ruyle, G. B. (1986). Influence of climatic and edaphic factors on the distribution of Eragrostis lehmanniana Nees. in Arizona USA. / Grassland Soc. South Afr. 3, 25-29. Cox, J. R., DeAlba-Avila, A., Rice, R. W., and Cox, J. N. (1993). Biological and physical factors influencing Acacia constricta and Prosopis velutina estabUshment in the Sonoran Desert. /. Range Mgmt 46, 43-48. Darkoh, M. B. K. (1998). The nature, causes and consequences of desertification in the drylands of Africa. Land Degrad. Dev. 9,1-20. Dean, W. R. J., Hoffman, M. T., Meadows, M. E., and Milton, S. J. (1995). Desertification in the semi-arid Karoo, South Africa: review and reassessment. /. Arid Environ. 30, 247-264. Derbyshire, E., Meng, X., and Kemp, R. A. (1998). Provenance, transport and characteristics of modem aeolian dust in western Gansu Province, China, and interpretation of the Quartemary loess record. J. Arid Environ. 39, 497-516. DeSoyza, A. G., Whitford, W. G., Virginia, R. A., and Reynolds, J. F. (1997a). Effects of summer drought on the water relations physiology and growth of large and small plants of Prosopis glandulosa and Larrea tridentata. In J. R. Barrow, E. D. McArthur, R. E. Sosebee, and R. J. Tausch (eds). Proceedings: Shrubland Ecosystem Dynamics in a Changing Environment, pp. 220-223. General Technical Report INT-GTR-338, US Department of Agriculture, Forest Service, Intermountain Research Station, Ogden, UT. DeSoyza, A. G., Whitford, W. G., Martinez-Meza, E., and Van Zee, J. W. (1997b), Variation in creosotebush {Larrea tridentata) canopy morphology in relation to habitat, soil fertility and associated annual plant communities. Am. Midi. Nat. 137, 13-26. El-Baz, F. (1983). A geological perspective of the desert. In S. Wells and D. Haragan (eds). Origin and Evolution of Deserts, pp. 163-183. University of New Mexico Press, Albuquerque. Fernandez, O. A. and Busso, C. A. (1999). Arid and semi-arid rangelands: two thirds of Argentina. In O. Amalds and S. Archer (eds). Case Studies of Rangeland Desertification: Proceeding from an International Workshop in Iceland, pp. 41-60. Rala Report 200. Agricultural Research Institute, Reykjavik, Iceland. Fullen, M. A. and Mitchell, D. J. (1994). Desertification and reclamation in north-central China. Ambio 23, 131-135. Gibbens, R. P. and Beck, R. F. (1988). Changes in grass basal area and forb densities over a 64-year period on grassland types of the Jornada Experimental Range. / Range Mgmt 41, 186-192. Gibbens, R. P., Tromble, J. M., Hennessy, J. T, and Cardenas, M. (1983). Soil movement in mesquite dunelands and former grasslands of southern New Mexico from 1933 to 1980. /. Range Mgmt 36, 145-148. Glendening, G. E. (1952). Some quantitative data on the increase of mesquite and cactus on a desert rangeland in southern Arizona. Ecology 33, 319-328. Gonzalez, C. L. (1990). Brush reinfestation following mechanical manipulation. /. Arid Environ. 18, 109-117. Griffith, L. W , Schuman, G. E., Rauzi, R, and Baumgartner, R. E. (1984). Mechanical renovation of shortgrass prairie for increased herbage production. J. Range Mgmt 27, A59-A62. Griffiths, D. (1901). Range improvement in Arizona. US Department of Agriculture, Bureau of Plant Industry, Bulletin No. 4. US Government Printing Office, Washington DC. Grove, A. T. (1986). Desertification in southern Europe. Clim. Change 9, 49-57. Grover, H. D. and Musick, H. B. (1990). Shrubland encroachment in southern New Mexico, USA: an analysis of desertification processes in the American southwest. Clim. Change 17, 305-330. Guo, Q. (1996). Effects of bannertail kangaroo rat mounds on small-scale plant community structure. Oecologia 106, 247-256. Harper, K. T., Van Buren, R., and Kitchen, S. G. (1996). Invasion of alien annuals and ecological consequences in salt desert shrublands of western Utah. In J. R. Barrow, E. D. McArthur, R. E. Sosebee, and R. J. Tausch (eds). Proceedings: Shrubland Ecosystem Dynamics in a Changing Environment, pp. 58-65. General Technical Report INT-GTR 338. US Department of Agriculture, Forest Service, Intermountain Research Station, Ogden, UT.

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Hastings, J. R. and Turner, R. M. (1965). The Changing Mile. University of Arizona Press, Tucson, AZ. Hennessy, J. T., Gibbens, R. R, Tromble, J. M., and Cardenas, M. (1983). Vegetation changes from 1935 to 1980 in mesquite dunelands and former grasslands of southern New Mexico. /. Range Mgmt 36, 310-314. Herbel, C. H., Gould, W. L., Leifeste, W. E, and Gibbens, R. R (1983). Herbicide treatment and vegetation response to treatment of mesquites in southern New Mexico. /. Range Mgmt 36,149-151. Holachek, J. L. (1992). Financial aspects of cattle production in the Chihuahuan Desert. Rangelands 14,145-149. Huenneke, L. F. and Noble, I. (1996). Ecosystem function of biodiversity in arid ecosystems. In H. A. Mooney, J.H. Cushman, E. Medina, O. E. Sala, and E.-D. Schulze (eds). Functional Roles of Biodiversity: A Global Perspective, pp. 99-128. John Wiley, London. Johnson, K. H., Vogt, K. A., Clark, H. J., Schmitz, O. J., and Vogt, D. J. (1996). Biodiversity and the productivity and stability of ecosystems. Tree 11, 312-311. Karrar, G. and Stiles, D. (1984). The global status and trend of desertification. / Arid Environ. 7, 309-313 Kassas, M. (1977). Arid and semi-arid lands: problems and prospects. Agro-Ecosystems 3,185-204. Kepner, W. G., Watts, C. J., Edmonds, C. M., Van Remortel, R. D., and Hamilton, M. E. (1999). A landscape approach for detecting and evaluating change in a semi-arid environment. Environ. Monit. Assess. 64, 179-195. Kerley, G. I. H. and Whitford, W G. (2000). Impact of grazing and desertification in the Chihuahuan Desert: plant communities, granivores and granivory. Am. Midi Nat. 144, 78-91. Kerley, G. I. H., Whitford, W. G., and Kay, F. R. (1997). Mechanisms for the keystone status of kangaroo rats: graminivory rather than granivory. Oecologia 111, 422-428. Kulas, C. M. (2001). The effects of Lehman lovegrass (Eragrostis lehmanniana) on semiarid ecosystems: biopedturbation as an ecosystem indicator. MS Thesis. New Mexico State University, Las Cruces, NM, 107 pp. Lockwood, J. A. and Lockwood, D. R. (1993). Catastrophe theory: a unified paradigm for rangeland ecosystem dynamics. /. Range Mgmt 46, 282-288. Ludwig, J. A. and Tongway, D. J. (1994). Small-scale resource heterogeneity in semi-arid landscapes. Pacific Conserv. Biol. 1, 201-208. Ludwig, J. A. and Tongway, D. J. (1995). Spatial organization of landscapes and its function in semiarid woodlands, Austraha. Lands. Ecol. 10, 51-63. MacDonald, B. C. T. and Melville, M. D. (1999). The impact of contour furrowing on chenopod patterned ground at Fowlers Gap, western New South Wales. J. Arid Environ. 41, 345-357. Miller, B., Ceballos, G., and Reading, R. (1994). The prairie dog and biotic diversity. Conserv. Biol. 8,677-681. Milton, S. J., Dean, W R. J., and DuPlessis, M. A. (1994). A conceptual model of rangeland degradation, the escalating cost of declining productivity. Bioscience 44, 70-76 Moorhead, D. L., Fisher, F. M., and Whitford, W G. (1988). Cover of spring annuals on nitrogen-rich kangaroo rat mounds in a Chihuahuan Desert grassland. Am. Midi. Nat. 120, 443-447. Mun, H. T. and Whitford, W. G. (1990). Factors affecting annual plant assemblages on banner-tailed kangaroo rat mounds. /. Arid Environ. 18, 165-173. Nash, M. S., Anderson, J. P., and Whitford, W. G. (1999). Spatial and temporal variability in relative abundance and foraging behavior of subterranean termites in desertified and relatively intact Chihuahuan Desert ecosystems. Appl. Soil Ecol. 12,149-157. Noble, J. C. (1997). The Delicate and Noxious Scrub: CSIRO Studies on Native Tree and Shrub Proliferation in the Semi-arid Woodlands of Eastern Australia. CSIRO Division of Wildlife and Ecology, Canberra. O'Conner, T. G. (1995). Acacia karroo invasions of grassland: environmental and biotic effects influencing seedling emergence and estabhshment. Oecologia 103, 214-223. Omar, S. A. S., and Abdal, M. (1994). Sustainable agricultural development and halting desertification in Kuwait. UN Environ. Programme Desertification Control Bull. 24, 23-26.

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Ovalle, C , Aronson, J., Avendano, J., Meneses, R., and Moreno, R. (1993). Rehabilitation of degraded ecosystems in central Chile and its relevance to the arid 'Norte Chico'. Rev. Chile. Hist. TVa/. 66,291-303. Ovalle, C , Aronson, J., Del Pozo, A., and Avendano, J. (1999). Restoration and rehabilitation of mixed espinales in central Chile: 10-year report and appraisal. Arid Soil Res. Rehabil. 13, 369-381. Pickup. G. (1985). The erosion cell - a geomorphic approach to landscape classification in range assessment. AM5^. Rangelands J. 7, 114-121. Pickup, G. and Chewings, V. H. (1986). Mapping and forecasting soil erosion patterns from Landsat on a microcomputer-based image-processing facility. Aust. Rangelands J. 8, 57-62. Rappole, J. H., Russell, C. E., Norwine, J. R., and Fullbright, T. E. (1986). Anthropogenic pressure and impacts on marginal, neotropical, semiarid ecosystems: the case of south Texas. Sci. Total Environ. 55, 91-99. Roundy, B. A. and Jordan, G. L. (1988). Vegetation changes in relation to livestock exclusion and root plowing in southeastern Arizona. Southwest. Nat. 33, 425-^36. Schlesinger, W. H., Reynolds, J. P., Cunningham, G. L., Huenneke, L. E, Jarrell, W. M., Virginia, R. A., and Whitford, W. G. (1990). Biological feedbacks in global desertification. Science 247, 1043-1048. Smith, D. A. and Schmutz, E. M. (1975). Vegetative changes on protected versus grazed desert grassland range in Arizona. J. Range Mgmt 28, 4 3 3 ^ 5 7 . Tongway, D. J. and Ludwig, J. A. (1990). Vegetation and soil patterning in semi-arid mulga lands of eastern Australia. Aust. J. Ecol. 15, 23-34. Van Auken, O. W. (2000). Shrub invasions of North American semiarid grasslands. Annu. Rev. Ecol. Syst. 31, 197-215. Van der Merwe, J. P. A. and Kellner, K. (1999). Soil disturbance and increase in species diversity during rehabilitation of degraded arid rangelands. J. Arid Environ. 41, 323-333. Verstraete, M. M. (1986). Defining desertification: a review. Clim. Change 9, 5-18. Weltzin, J. E, Archer, S., and Heitschmidt, R. K. (1997). Small-mammal regulation of vegetation structure in a temperate savanna. Ecology 78, 751-763. Westoby, M., Walker, B., and Noy Meir, I. (1989). Range management on the basis of a model which does not seek to establish equilibrium. /. Arid Environ. 17, 235-239. Whitford, W. G. (1993). Animal feedbacks in desertification: an overview. Rev. Chil. Hist. Nat. 66, 243-251. Whitford, W. G. (1996) The importance of the biodiversity of soil biota in arid ecosystems. Biodiversity Conserv. 5, 185-195. Whitford, W G. (1997). Desertification and animal biodiversity in the desert grasslands of North America. /. Arid Environ. 37, 709-720. Whitford, W. G., Martinez-Turanzas, G., and Martinez-Meza, E. (1995). Persistence of desertified ecosystems: explanations and implications. Environ. Monitor Assess. TH^ 319-333. Whitford, W. G., Nielson, R., and DeSoyza, A. G. (2001). Establishment and effects of estabhshment of creosotebush, Larrea tridentata, on a Chihuahuan Desert watershed. /. Arid Environ. 47, 1-10. Williams, M. A. J. and Balling, R. C. Jr. (1996). Interactions of Desertification and Climate. Arnold, London. York, J. C. and Dick-Peddie, W A. (1969). Vegetation changes in southern New Mexico during the past hundred years. In W. G. McGinnies and B. J. Goldman (eds). Arid Lands in Perspective, pp. 155-166. The University of Arizona Press, Tucson, AZ.

Chapter 11

Monitoring and Assessment

The widespread continuing degradation of arid and semiarid lands has led to recognition of the need for monitoring and assessment systems to detect degradation trends and serve as early warning systems. The shift from a 'functional' ecosystem or landscape to a 'dysfunctional' ecosystem or landscape may be effectively irreversible when the system has moved past some 'threshold'. Monitoring and assessment systems need to be able to detect degradation trajectories and to provide indicators that allow identification of thresholds of irreversible change. Despite the perception that monitoring and assessment systems are needed, there is no general agreement as to what should be monitored or how monitoring results should be reported or interpreted. What are the goals of monitoring and assessing the status of arid and semiarid lands? What are the values that society places on arid and semiarid lands? Can these values be translated into measurable goals for a monitoring and assessment program? These are questions for which there are no answers that are generally applicable to arid and semiarid lands around the world. Land managers seek monitoring and/or assessment systems that can provide information on the spectrum of goals expressed by various groups within society. The information required of monitoring and/or assessment systems is different for the various user groups in society. Each user group espouses different values for arid and semiarid lands. Monitoring and assessment systems must therefore provide sufficient data to describe the state of the system with respect to the divergent values of the user groups. Government agencies call for assessments of the status of lands encompassed by political boundaries. Political boundaries rarely coincide with regional drainage systems or with functional landscapes. However, the status of the lands within political boundaries can only be assessed within a proper geographical context. In arid lands this is frequently a difficult problem because many of the environmental stressors affecting the sustainability of regional ecosystems originate outside the political region. Calls for assessments are usually vague particularly with regard to the standards against which the status of the lands are to be measured. The standards against which assessments are compared are often some idealized 'pristine' or relatively 'undisturbed' concept of what the ecosystems in a region should resemble. Since all biological communities and ecosystems change both structurally and functionally

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over time in response to variation in climate and other abiotic variables, it is essential to factor out 'normal ecological change' from changes resulting from anthropogenic stresses imposed on ecosystems. Because of the natural temporal changes in structure and function of ecosystems, monitoring studies are essential to provide the natural background 'standard' for assessments.

11.1 DEFINITIONS Assessment is a one-time process that relies on a statistically valid sampling within a region. An assessment system provides data on the status of the ecosystems of a region and should be used across broad geographical areas. The measurements made in an assessment system must be rapid, and sufficiently simple to be made by persons with limited experience and expertise. Assessment measurements are converted into indicators by combining a group of measurements or by calculating indicator values from the basic measurements. Indicators should be measures of ecosystem properties that are related to ecosystem processes in some documented way. A suite of such indicators provides information on how well the ecosystem(s) are performing selected functions (such as conservation of water and nutrients). If assessments are made repeatedly at the same locations, the assessment indicators can be incorporated into a monitoring system in order to ascertain the trends in ecosystems of a region. Data on landscape characteristics obtained by remote sensing have considerable potential for assessing condition. Monitoring consists of a series of measurements designed to provide information on the trajectory of change in ecosystem structure and/or function over time. Monitoring systems are not limited by the same time constraints that are imposed on assessments. Ideally monitoring involves repeated measurements on the same location at fixed time intervals. Monitoring sites need not be selected to fit a particular statistical model. Monitoring sites should be selected on the basis of a set of criteria or questions that may differ from region to region and even among locations within a region. For example, a land manager may want to know how effective a management protocol is at restoring ecosystem function that has been recognized as degraded or lost. The manager must then decide what landscape locations will provide the information necessary to answer that question. The site locations and measurements selected should be chosen to answer one or more questions about status and trends of the ecosystems of a landscape unit.

11.2 EXISTING ASSESSMENT AND MONITORING SYSTEMS Assessment and monitoring systems for arid rangelands are described in monitoring manuals for a variety of Australian rangelands (Tongway, 1994; Tongway and Hindley, 1995) and South African Karoo desert rangelands (Milton and Dean,

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1996). Although the Australian and South African rangelands differ in a number of very important ways, they also share a number of the same attributes. The monitoring systems that have been proposed for these regions use similar indicators that are related to the same processes. In the United States, new methods to inventory, classify, and monitor semiarid and arid rangelands (lands used for livestock production) were described in the US National Research Council pubUcation, Rangeland Health (National Research Council, 1994). The Karoo Desert manual is designed primarily for the land manager (farmers) in the Karoo. The assessment system is qualitative but that allows semiquantitative evaluation of some of the most important variables. The Karoo system scores attributes of the system from 5 to 1 with 5 being highest quality and 1 lowest quality. The attributes measured or estimated include forage value, use of palatable plant species, alien weeds, the ratio of palatable:unpalatable species of seedlings of perennial plants, and soil health. The soil health variables include those that are positive (mulch, digging, shade plants) and those that are negative (capping, pedestals, rills, roots). The soil health variables are rated common to uncommon. Uncommon is good for the negative soil health variables and common is good for the positive soil health variables (Milton and Dean, 1996). The Australian soil condition assessment manuals focus on the hypothesis that arid land ecosystems should retain water and nutrient resources. The assessment and monitoring systems are focused at the landscape scale. This scale was selected because arid and semiarid systems frequently function as 'source'-'sink' systems that are linked at the landscape scale. The indicators selected for assessing soil condition are variables that affect the retention of water and nutrients within the landscape. Vegetation is evaluated as to its obstruction value - resisting flow, reducing velocity of overland flow and causing sediment deposition. Soil surface stability and resistance to splash erosion are evaluated by a simple field measure of aggregate stability and crust characteristics (presence/absence and percentage cover of cryptogamic crusts (biological crusts composed of algae, lichens, and fungi). The soil condition assessment system can be converted into a monitoring system by establishing permanent line transects on which vegetation and soil measurements are made at a predetermined frequency (Tongway, 1994; Tongway and Hindley, 1995). The assessment of rangeland health focused on qualitative indicators. For example, soil condition is assessed by the presence or absence of an A horizon, rills and guUies, soil pedestahng, evidence of scouring and sheet erosion, and dune formation. Biotic integrity is assessed by plant distribution, litter distribution, rooting depth, photosynthetic period, age class, plant vigor, and germination microsites (National Research Council, 1994). The rangeland health indicators are rated healthy, at risk and unhealthy by qualitative evaluations that require considerable expertise on the part of the evaluator (Table 11.1). Some of the indicators (plant vigor, rills) require completely subjective evaluation and it is nearly impossible for a group of trained observers to agree on the state of these indicators, i.e. healthy, at risk, or unhealthy. Three categories of ecosystem attributes related to

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materials must be harvested from local regions and processed by the human populations within the region. This is a critical issue for countries where most of the land area is arid or semiarid. One of the more successful programs for development of new crops for arid zones has been at the Ben-Gurion University of the Negev. That program was designed to attract settlement of the desert by providing means of production that offered sufficient income to support a reasonable standard of living within the constraints of limited water supply (Pasternak et aL, 1986). Some desert plants are the basis for cottage industries in a number of countries. Among the most economically important species are the Cactaceae. One species {Opuntia ficus-indica) was an important commercial plant in the Mediterranean region of Europe and North Africa during the 18th century where it was cultivated to grow cochineal insects for a source of dye. The genus Opuntia is widely used as fodder, forage, fruit, and as a green vegetable especially in Mexico but also in many other parts of the world (Russell and Felker, 1987). Cactaceae use crassulacean acid metabolism which can be up to five times more efficient in converting water into dry matter than the most efficient grasses. Some of the domesticated strains of Opuntia have fresh fruit yields of 8000-12,000 kg ha-i yr'^ and dry matter production of 20,000-50,000 kg ha-^ yr~^ (Russell and Felker, 1987). The fruits of Opuntia cultivars are sold in markets in the US, Chile, Mexico, Brazil, North Africa, and Mediterranean countries. The young cladodes (pads) are used extensively as a fresh green vegetable in Mexico and the border states of the US. Research on methods of processing cactus fruits and cladodes has documented a number of food-processing technologies that can increase the shelf life of cactus products. The fruits can be processed into juices, marmalades, gels, and liquid sweetener. The cladodes can be pickled and processed into food additives (Saenz, 2000). Other uses of cactus fruits and cladodes, such as food coloring and medical uses, are currently being explored. In Mexico and Argentina, Opuntia spp. are planted in plantations, often on marginal lands, where they are harvested for human food and for animal fodder. Cactus production was found to be feasible in a direct browsing system on a 100 ha plantation in a 300-mm rainfall region and on a 50 ha plantation in a 400-mm rainfall region. It was estimated that the establishment costs of such plantations would increase the investment costs of the stockman by 7.4-10% (Guevara et aL, 1999). Local dairy operators consider cactus pad fodder important for lactation of their cows and for the quality of the milk they produce (Russell and Felker, 1987). These authors suggest that prickly pear (Opuntia spp.) can be grown as a fodder crop on lands that are marginal for grain production. Opuntia can be successfully intercropped with plants, such as mesquite (Prosopis spp.) that enrich the soil and increase the productivity of the prickly pear. Because prickly pear are important forage for game animals, such as deer (Odocoileus virginianus), javelina (Pecari tajacu) and several species of game birds in arid and semiarid regions, they provide the basis for recreational hunting. In Texas, prickly pear-mesquite rangeland produces two to three times more income per hectare

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from recreational hunting permits than from cattle grazing (Russell and Felker, 1987). There are many other desert plants that could be cultivated to produce food and fiber in arid environments. Becker (1986) identified several plants native to the Sahel that are important as 'wild' food for the humans in the region. These plants provide important sources of vitamins and minerals. Many species of trees and shrubs produce fruits and seeds that are edible. These species are more reliable as sources of food than are annual plants. Several of the species listed by Becker (1986) appear to be good candidates for plantation or orchard crop plants. The large volume of brackish or saline water that is available in some arid regions is a resource that can potentially be utilized for production of food crops (O'Leary, 1987). The most promising 'new' crops that could be produced using saline water are halophytic seed crops. The foliage of halophytic species such as Atriplex spp. contain salt as a high percentage of the dry mass. Livestock production utihzing the foliage of halophytic species requires fodder produced by fresh water in order to balance the use of the halophytic foliage. However, the salt content of halophyte seeds is no higher than that of seeds of nonhalophytic plants. There are many species of arid zone plants that produce fibers with desirable characteristics. For example, hard fibers are tissues obtained by decorticating leaves of a variety of monocotyledonous plants. These fibers have traditionally been used in the manufacture of rope and burlap but are also pulped and used in the manufacture of specialty papers including currency, tea bags, and other products that require high tensile strength. The fiber characteristics of several genera of Agavaceae that occur in North American hot deserts were examined to determine their paper-making qualities (McLaughin and Schuck, 1991). Several species had fibers that were as long as or longer and smaller in diameter than the fibers of sisal (primary fiber for rope manufacture). Species in two genera, Hesperaloe and Yucca appeared to be most suitable for manufacturing paper. Another example of an arid land plant that provides fiber for a native plant industry is the palmilla (Nolina spp.). The leaves are split, trimmed, dried, and bundled for sale to broom-manufacturing companies. Leaves may be harvested from the same population of plants every 18-24 months which provides a sustainable harvest of an arid land plant (Nabhan and Bums, 1985). The potential for meat production in arid and semiarid lands has not been realized because of the focus on husbandry of cattle and sheep. These domesticated species have few if any physiological adaptations to arid environments. The one exception is the camel (Camelus spp.) which has been used extensively as a source of milk products, fiber for clothing, meat, and for transporting materials and humans. The desertification problem is attributable in large part to the replacement of native herbivores with domestic species. This problem was eloquently addressed by Eraser Darling (1960) who said in reference to African deserts and savannas, 'to exchange the wide spectrum of 20-30 hoofed animals, living in delicate adjustment to their habitat, for the narrowed spectrum of three ungulates exotic to Africa - cattle, sheep and goats - is to throw away a bountiful resource


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and marvelous ordering of nature.' Fraser Darling's statement applies (with modification of number of native herbivores) to the arid lands of the America's, Australia, and Asia. There have been no organized efforts to domesticate the several species of African antelopes that are physiologically adapted to arid environments. There is growing evidence that much greater meat and fiber production can be obtained from arid-adapted animals with much less stress on the arid ecosystems than from present-day agricultural enterprises. One example is from an introduction of a desert-adapted species (the South African gemsbok, Oryx gazella) as a source of recreational hunting in the Chihuahuan Desert of New Mexico. From a few animals introduced into the Tulorosa Basin in the 1970s, these animals now number in the thousands and have spread into large areas off the reserve where they were originally introduced. Because Oryx do not use large amounts of water for thermoregulation, they are able to forage over large areas where surface water is not available. There is increasing evidence that Oryx in New Mexico have higher rates of reproduction and higher survivorship of calves than the source populations in Southern Africa. Arid-adapted animals like Oryx are not constrained by distance from surface water and therefore are able to distribute their grazing-browsing activity more evenly across the landscape than domestic cattle or sheep. Despite the success of this species in Chihuahuan Desert shrublands, there are increasing calls for elimination of the species from grazing lands because they are 'exotic'. The rationale is that Oryx compete with domestic livestock (which are really exotic animals in a desert environment). Because of cultural bias, it is difficult to convince the public to consider cattle and sheep as exotics in desert environments. In Africa, cattle account for approximately two-thirds of the biomass of domestic livestock. Cattle because of their Eurasian origin have neither the physiology nor grazing behavior adapted to the arid and semiarid pastoral lands of Africa. This is especially apparent during drought when cattle are malnourished because they overgraze areas that are within their range from water (Kay, 1997). Because cattle cannot range far from water and are dependent on drought-susceptible grasses rather than on the more drought-resistant trees and shrubs, overgrazing contributes to desertification. Many of the native African herbivores are capable of moving long distances from water and are facultative browsers when grass biomass is low. Despite the documentation of the adaptations of red and gray kangaroos to life in arid environments, most of the arid and semiarid rangelands of Australia are still devoted to cattle and sheep pastoralism. Several scientists have made the case for replacing sheep and cattle pastoraHsm with management and harvest of native kangaroos and other native marsupials. These animals, even in high numbers, have a much lower impact on the arid and semiarid rangelands than sheep or cattle. Because arid-adapted animals are not tied to water sources, there is reduced impact of these herbivores in the area around water points. A reduction in activity that produces piospheres around water points is an important consideration in

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reducing or stopping desertification. Unless the meat production industry in the arid regions of the world dramatically changes their management systems, degradation of rangelands will continue unabated. Exchanging cattle and sheep that are cool, mesic climate-adapted species, for native or nonnative herbivores that are desert adapted could be an important step in halting and even reversing the degradation trends in the semiarid regions of the world. In most deserts, termites appear to process a large fraction of the total litter input. Because termites are social insects, their numbers and biomass far exceeds that of other detritivorous macroarthropods in most deserts. High biomass of subterranean and epigeic termites have been reported for semiarid savanna in southern Africa (Ferrar, 1982) and large populations have been reported from Saudi Arabia (Badawi et ai, 1984). Johnson and Wood (1980) report on a number of species of termites that occur in the arid regions of Africa and the Arabian Peninsula. Their summary of the feeding habits of the species shows that in most of these regions, dead roots and above-ground herbaceous and woody vegetation are utilized by one or more of the species. I have observed large numbers of harvester termites {Hodotermes mossambicus) cutting stems of dead grasses or collecting stems and leaves of grasses in the Kalahari Desert and in parts of the Karoo Desert in South Africa. The abundance of soil dumps and foraging entrances in these areas suggest that termites probably process a large fraction of the standing dead grass and detritus in these deserts. Because such a large fraction of the primary production in most deserts is converted into termite biomass, it would seem logical to develop ways in which termites can be converted into food for humans. The high nutrient content of termites and high rates of population growth by some species led to a proposal that termites be harvested to provide high protein supplement for chickens or other domestic stock (French, 1982). French (1982) proposed that termites could be attracted into baits that could be removed from epigeic mounds or collected from the soil surface. The termites could be emptied from the bait containers into the chicken feed trays. French's proposal could become an important component of food production in arid regions where only a small fraction of the net primary production can be converted into consumable biomass by domesticated animal species. In South Africa where domesticated ostrich (Struthio camelus) are an important part of the agricultural industry, termites have been shown to be a suitable protein supplement to the vegetable matter diet typically used for raising ostrich chicks (Milton et ai, 1993). Utilizing termites to feed chickens or other domesticated animals can provide a mechanism for harvesting a larger fraction of the net primary production and for converting unusable cellulose into food for human consumption. Sustainable use of arid lands by humans requires thinking 'outside the box' and a willingness to explore nontraditional uses of arid lands. Sustainable use requires that the ecosystem services provided by arid lands are maintained and not sacrificed to short-term goods production. Management of arid lands for sustainabihty


325

requires an understanding of ecosystem properties and processes and how these interact with the unique conditions of arid environments and arid landscapes.

REFERENCES Badawi, A., Faragalla, A. A., and Dabbour, A. (1984). Population studies of some species of termites in Al-Kharji oasis, central region of Saudi Arabia. Z Angewandte Entomol. 97, 253-261. Becker, B. (1986). Wild plants for human nutrition in the Sahelian Zone. /. Arid Environ. 11, 61-64. Costanza, R., d'Arge, R., de Groots, R., Farber, S., Grasso, M., Hannon, B., Limburg, K., Naeem, S., O'Neil, R. v., Paruelo, J., Raskin, R. G., Sutton, P., and ven den Belt, M. (1997). The value of the world's ecosystem services and natural capital. Nature 387, 253-260. Ferrar, P. (1982). Termites of a South African savanna. II. Densities and populations of smaller mounds, and seasonality of breeding. Oecologia 52, 113-138. Fraser DarHng, F. (1960). Wildlife in an African Territory. Oxford University Press, Oxford, UK. French, J. (1982). Termites can be chicken feed. Grass Roots 32, 50-51. Guevara, J. C., Estevez, O. R., and Stasi, C. R. (1999). Economic feasibility of cactus plantations for forage and fodder production in the Mendoza plains (Argentina). /. Arid Environ. 43, 241-249. Hinman, C. W. (1984). New crops for arid lands. Science 225, 1445-1448. Johnson, R. A. and Wood, T. G. (1980). Termites of the arid zones of Africa and the Arabian Peninsula. Sociobiology 5, 279-293. Kay, R. N. B. (1997). Responses of African livestock and wild herbivores to drought. J. Arid Environ. 37, 683-694. McLaughlin, S. P. and Schuck, S. M. (1991). Fiber properties of several species of Agavaceae from the southwestern United States and northern Mexico. Econ. Bot. 45, 4 8 0 ^ 8 6 . Milton, S. J., Dean, W. R. J., and Linton, A. (1993). Consumption of termites by captive ostrich chicks. South Afr J. Wildl. Res. 23, 58-60. Nabhan, G. B. and Burns, B. T. (1985). Palmilla (Molina) fiber: a native plant industry in arid and semi-arid U.S./Mexico borderlands. J. Arid Environ. 9, 97-103. O'Leary, J. W. (1987). Halophytic food crops for arid lands. In E. F. Aldon, C. E. Gonzales-Vincente, and W. H. Moir (eds). Strategies for Classification and Management of Native Vegetation for Food Production in Arid Zones, pp. 1-5. General Technical Report RM 150, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, Co. Pasternak, D., Aronson, J. A., Ben-Dov, J., Forti, M., Mendlinger, S., Nerd, A., and Sitton, D. (1986). Development of new arid zone crops for the Negev Desert of Israel. /. Arid Environ. 11, 37-59. Russell, C. E. and Felker, P. (1987). The prickley-pears (Opuntia spp., Cactaceae): a source of human and animal food in semiarid regions. Econ. Bot. 41, 433-445. Saenz, C. (2000). Processing technologies: an alternative for cactus pear (Opuntia spp.) fruits and cladodes. /. Arid Environments 46, 209-225.

Index

Aardvark {Orycteropus afer), 100 Acacia, 22, 38, 103, 189, 190, 206, 254, 260 see also Mulga woodland Acacia aneura, 34, 200 see also Mulga woodland Acacia caven, 254, 277, 300 Acacia constricta, 113 Acacia erioloba, 101 Acacia karoo, 287 Acacia schajfneri, 104 Acari, soil meso/microfauna, 221 anhydrobiosis/cryptobiosis, 247 decomposition activities, 248 soil quality indicator, 314 Actinomycete nitrogen fixation, 260 Adaptations, 123-151 amphibians, 126-129 annual plants, 124-125 arthropods, 131-133, 142-143 avoidance of extremes, 123-133 birds, 131,150-151 carnivores, 150 mammals, 129-131, 143-150 perennial plants, 125-126,133-151 physiological/morphological, 133-151 reptiles, 129, 141-142 soil mesofauna, 143 Advanced Very High Resolution Radiometer (AVHRR) remote sensing system, 314-315 Aedes, 223 Aeolian features, 37-38 classification, 38 Aepyceros melampus (impala), 189 Agavaceae, 322 Agavae, 105 Agave deserti, 138 Algal crusts nitrogen fixation, 259 resistance to wind erosion, 68

soil nutrient enrichment, 259-260 see also Crytogamic crusts Alkaloids, 185, 186, 192, 195 Alluvial fans, 25-30 Ambrosia, 140, 174 Ambrosia deltoidea, 107 Ambrosia dumosa, 105, 107 Ammonia volatilization, 262-263 Amphibians, 223,224 adaptations, 126-129 Anabasis, 22, 138 Anhydrobiosis, 143,247 Animal activities disturbance patches, 110-117, 184, 214 nutrient heterogeneity, 253 temporal changes, 118 pits as seed traps/germination sites, 98-99, 100-101, 184 rainfall infiltration effects, 82-84, 184 seasonality of reproduction, 48 seed dispersal, 101-102 soil disturbance, 110 macropore formation, 82-84,110 wind erosion susceptibility, 68-69, 70 Animal adaptations see Adaptations Animal indicators for monitoring/assessment, 314 Annual above ground net primary production, 157 comparisons with mesic ecosystems, 160-161 landscape relationships, 166-170 measurement, 158-159 models, 163-166 rainfall relationship, 163-164, 165, 166, 167, 168, 170, 171-172 Annual plants adaptations, 124-125 rainfall seasonality relationships, 50 rooting systems, 254 seed dormancy polymorphism, 124-125

328

Index

Antelope, 323 Anthyllis henoniana, 140 Antioxidant feeding deterents, 186 Ants, 198, 199, 201-202, 217, 268 behavioural adaptations, 133 granivory, 207,208,211,212,213 impact on ecosystem structure/processes, 213-215 indicators for monitoring/assessment, 313, 314 lizard predation, 218 omnivory, 185 plant productivity impact, 113 soil effects, 113, 114-117 macropore formation, 84,110 nutrients, 110, 113 Aplomodo falcon (Falco femoralis), 295 Apparent competition, 185 Arabian desert, 21-22 aridity index (I^), 46 insect communities, 198,199 seasonality, 49, 51 Arachnids, 133 Arenivaga, 131 Arid regions, 1 definition, 12 water balance parameters, 46-48 Aridisols, 15 Aridity classification, 12 definition, 11-12 Aridity index (/^^), 46-47 Aristida, 141 Aristidapungens, 166 Arroyos see Ephemeral streams Artemisia, 22,314 Artemisia campestris, 140 Artemisia herba-alba, 140, 168, 169 Artemisia monosperma, 37 Artemisia tridentata (ssLgobrush), 107, 188 Arthropods adaptations, 131-133 canopy insect communities, 192-198 herbivores leaf chewing, 190 sucking, 193, 194 mycorrizal spore dispersal, 268 plant productivity impact, 113 secondary production, 182 soil effects, 113, 114-117 macropore formation, 87, 110 soil fauna, 220-221 soil microfauna

decomposition activity, 247-248, 251 nitrogen mineralization, 264 soil quality indicator, 314 surface active, 198-202 Asphodelus ramosus, 196 Assemblages, 10 Assessment systems see Monitoring/assessment systems Astragalus, 22, 186, 260 Atalaya hemiglauca, 35 Atmospheric layer, hydrological landscape, 73 Atriplex, 29,76,322 Atriplex canescens, 266, 269 Atriplex confertifolia, 53, 149, 162 Atriplex poly carp a, 29 Atriplex vesicaria, 281 Australian desert aridity index (/^), 48 desertification, 276, 279, 287 desertified landscape rehabihtation, 298 drought, 44-45 dune field vegetation, 39 lizard communities, 215 monitoring/assessment systems, 306, 307, 309 nutrient availability, 236 rodent granivory, 209, 210, 211 seasonality, 49 surface active arthropods, 199-200 Australian dotterel, 151 Australian netted dragon (Cnemidophorus nuchalis), 217 Autoecological hypothesis, 12-13, 14 Azobacter, 259 Azospirillum, 259, 261 Azotobacter, 261 B Baccharis, 195 Bacterial decomposition, 249 Badger (Taxidea taxus), 206 Bajadas, 25-30 Banded vegetation patterns, 33, 118-119 Banner-tail kangaroo rat {Dipodomys spectabilis), 129,288,313 Barkhan (crescent-shaped) dunes, 38 Basins, 21,22,33-36 Bat-eared fox {Otocyon megalotis), 100 Bedouin goat, 147 Beetles, 133, 198, 199, 217 Behavioural adaptations, 123 arthropods, 131-133 birds, 131

Index mammals, 129-131 Below-ground food webs, 220-222 Below-ground productivity, 162 Bettongia lesuer. 111 Bioclimatic aridity indices, 11 Biodiversity, 310,313 adaptations to regional climate, 45 desertification impact, 293-295 Biomass estimates, 157-159 rootbiomass, 162 Biome, 2 adaptations to seasonality, 45 Biosphere, 2 Biotic interactions, 6-7, 17 Biotic-abiotic interactions, 9-10, 17 Birds, 313 adaptations behavioural, 131 physiological, 150-151 granivory, 207 nomadism, 131 omnivory, 185 secondary production, 181 seed/fruit dispersal, 99,101,206 Black grama (Bouteloua eriopoda), 69, 117, 141,235-236,284,297 Black-tail jackrabbit (Lepus californicus), 186, 188 Body temperature lability, large mammals, 144-145 Bootettix punctatus, 181 Boulder-strewn surfaces, 22, 36 Bouteloua eriopoda (black grama), 69, 117, 141,235-236,284,297 Brachystomella arida, 200 Bradyhizobium, 260 Brain countercurrent heat-exchange system, 145 Brickellia lanciniata, 32, 257 Brittlebush (Encelia farinosa), 126 Bromus rubens, 288 Bromus tectorum (cheat grass), 287, 314 'Brousse tigree' vegetation, 33 Browsing, 186, 188-190, 191 plant responses, 204 Bruchid beetles, 207 Bufo cognatus, 224 Bugs, 217 Bunch grasses, 5, 6 shield/platform desert, 22 weather interface, 5 Burr sage {Ambrosia), 105, 107 Burr sage (Franseria), 29, 76

329

Burrowing activities, 110,184 amphibians, 126 arthropods, 131 spiders/scoipions, 200 mammals, 129-130 nutrient transport, 110, 253 Buttes, 24 C C3 photosynthetic pathway, 134-135, 138, 141 C4 photosynthetic pathway, 134-135, 138, 141 Cache pits, 100, 111,214 Cactaceae, 137, 138 commercial exploitation, 321 Cactus wren, 101, 102 Calcium, 117 Calcrete, 36 CAM (crassulacean acid metabolism) photosynthetic pathway, 134, 137, 138, 321 Camel (Camelus), 143, 144, 146-147 Camel cricket, 198 Canis latrans (coyote), 150, 185, 206, 288 Canis lupus (lobo wolf), 288 Canopy insect communities, 192-198 morphology, 5 Cape fox {Vulpes chama), 100 Cape ground squirrel (Xerus inauris), 130 Cape hare, 186 Capsodes infuscatus, 196 Carbohydrate exudation, 5 Carbon-nitrogen ratios, 238 Camegia, 138 Camegia gigantea (saguaro), 27, 29, 105, 107, 109, 288 Carnivores adaptations, 150 omnivory, 185 seed dispersal, 206 Cassida, 195 Casuarina cunninghamiana, 260 Catenas fluvial transport, 84-85 see also Watersheds Cell, 2 Celtis pallida, 99 Cenchrus ciliare, 288 Centipedes, 132,217 Ceratoides lanata (winterfat), 53, 162, 188 Cercidium (palo verde), 105, 173 Cercidium microphyllum, 29, 80, 107, 109 'Chaco' woodland, 166

330

Index

Cheat grass (Bromus tectorum), 287, 314 Chenopod shmbland, banded vegetation patterns, 33 Chihuahuan Desert bird-mediated seed dispersal, 101-102 decomposition processes, 244, 245 desertification changes, 284, 286 ephemeral plant seed production, 124 ephemeral pond/lake fauna, 223, 224, 225, 226 ephemeral streams, 32, 92 granivory, 207-208 herbivory, 204 indicators for monitoring/assessment, 312-313 multi-species patches, 104-105 nitrogen availability, 235 productivity, 161, 163, 166, 177 rainfall, 56,57 spatial variability, 52 seasonality, 46, 50, 51 animal reproduction, 48, 50 secondary production/herbivory, 182, 186, 196 seed densities, 98 surface active arthropods, 198, 199, 200, 201 termite activities, 83, 117, 249, 250 vegetation, 32, 74, 79-80 wind erosion, 69, 71 Chilopsis linearis (desert willow), 32, 257 China desertified landscape rehabilitation, 299 drought, 44 Chiromantis, 127 Chottes see Ephemeral ponds/lakes Chrysothamnus viscidiflorus (green rabbitbush), 188 C\mck}^di\\di {Sauromalus obesus), 141, 142 Cladodes, 137 Clam shrimp (Eulimnadia texana), 223 Climate, 43-61 seasonalality, 4 5 ^ 6 , 48-52 spatial effects, 52-53 water balance parameters, 46-48 see also Rainfall Climate change, desertification processes, 290-291 Clostridium, 261 Cnemidophorus, 217 Cnemidophorus tigris, 216 Cockroaches, 198, 199, 217 Cocoon formation, 127

Collared peccary (Pecari tajuca), 206, 321 Collembola, 143, 200, 221 anhydrobiosis, 247 Colorado River, 37 CommensaHsm, 107, 109 nurse plant relationships, 107, 109 Community, 2-3, 7 Compensatory growth, 188, 191 Competition, 10,17,106-110 apparent, 185 patch structure effects, 97 Consumers, 181-185 Convectional heat flux, 144 Convectional rainfall, 52 Coppice dunes, 276, 286 Corvus cryptoleucus (wite-necked raven), 150 Cottontails, 186 Coyote {Canis latrans), 150, 185, 206, 288 Creosotebush (Larrea tridentata), 3, 29, 51, 57, 70, 71, 74, 75, 76, 80, 103, 105, 106, 107, 109, 117, 198, 199, 201, 207, 217, 235,238,257,284,290,315 adaptations, 135-137 feeding deterents, 186 herbivory, 186, 188, 193, 196, 197 primary productivity, 173-174, 175 Crepuscular activity cycle, 130 Crescent-shaped (barkhan) dunes, 38 Crickets, 198, 199, 217, 268 Cryptobiosis, 143 Cryptogamic crusts disruption by animal activities, 82 nitrogen fixation, 259 rainfall infiltration effects, 81 soil nutrient enrichment, 259-260 see also Surface crusts Cryptopygus ambus, 200 Ctenophorus isolepis, 217 Ctenophorus nuchalis (Australian netted dragon), 217 Ctenotus, 216, 217 Cyanobacterial nitrogen fixation, 258, 259 Cynomys ludovicianus (prairie dog), 288, 289 D Dalea, 260 Dark septate fungal-root associations, 268-269 Dasyochloa (Erioneuron) pulchella, 141 Decomposition, 237-252 conceptual models, 251-252 fragmentation processes, 237, 238, 243, 251 litter, 238-239

Index rates/mechanisms of mass loss, 239-244 soil processes, 245-247,251 mineralization processes, 237-238, 243 nitrogen mineralization, 264-265 photodegradation processes, 240, 251 rainfall relationship, 240, 243, 244, 245, 247 root materials, 245-247, 251, 264 soil microfauna activities, 247-249, 251 spatial variation, 244 termite activities, 249-251 Definition of deserts, 11-12 Defoliation, 196 compensatory growth response, 202-203, 204 Degu (Octodon degus), 214 Denitrification, 262, 263-266 Desert cottontail (Sylvilagus auduboni), 188 Desert iguana {Dipsosaurus dorsalis), 141, 215 Desert tortoise {Gopherus agassizii), 142 Desert willow (Chilopsis linearis), 32, 257 Desertification, 9, 275-301, 322, 324 biodiversity impact, 293-295 climate change, 290-291 definition, 275 dynamic conceptual models, 282-283 ecosystem/landscape consequences, 282-289 environmental stressors, 275, 283 grazing-related changes, 278, 279, 280-282, 283-284 history, 277-282 landscape rehabilitation, 295-301 landscape resistance/resilience, 289-290 landscape structure changes, 280 loss/replacement of forage species, 276-277, 284, 286, 287 monitoring/assessment systems, 305 social/economic consequences, 291-293 spread of nonnative species, 287-288 tree/shrub harvesting, 282 Detritus-based food webs sand dune arthropods, 198 soil, 221,222 Dinothrombium, 133 Dipodomys (kangaroo rat), 45, 145, 149, 213, 217-218 Dipodomys merriami, 210, 213 Dipodomys microps, 149 Dipodomys ordii, 149, 213 Dipodomys spectabilis (banner-tail kangaroo rat), 129,288,313

331

Dipsosaurus dorsalis (desert iguana), 141, 215 Dolichotis patagonum, 206 Domestic livestock, 322, 323 seed dispersal into grassland, 206 Dominant species, 3, 5, 9 longevity, 105 physiological threshold limits, 55 Dorcas gazelle (Gazella dorcas), 190 Dromarius novaehollandiae (emu), 206 Drought, 43,312 change in habitat use, 45 decomposition processes, 244 El Niiio-Southem Oscillation, 44 impact on humans, 276 recovery period, 5-6 simulation experiments, 175-176 tropical sea surface temperature relationship, 43,44 Drought-deciduous perennials, 125-126 Dryfall, 261-262 Dust storms, 65

Earthworms, 220 soil macropore formation, 82 Echinocereus, 109 Economic impact of desertification, 276, 277, 291-293 Ecosystem, 2, 3 definition, 6-9 desertification consequences, 282-289 function, 7-8 health indicators, 307, 308-309 integrity, 310 structure, 2, 3-4, 7 Ecosystem engineering, 184 Ecosystem processes, 6,16-17 consumer effects, 184-185 human management issues, 8 monitoring/assessment, 309,310 scaling problems, 18 El Nino-Southern Oscillation long-term records, 45 rainfall effects, 43-45 Embryonic diapause, 147 Emu (Dromarius novaehollandiae), 206 Encelia farinosa (brittlebush), 126 Energy flow, 6, 7, 16, 181 consumer effects, 184,185 Entomobrya, 200 Ephedra torreyana (Torrey ephedra), 186 Ephedra trifurca (mormon tea), 186

332

Index

Ephemeral plants nitrogen-fixing legumes, 255 patchy distribution, 99 productivity, 167, 169 rainfall seasonahty correlations, 50 seed germination delay/inhibition, 124-125 see also Annual plants Ephemeral ponds/lakes, 90-92 aquatic fauna, 222-226 flooding cycle, 92 food webs, 92 Ephemeral streams, 30-32, 37, 166 stream bed vegetation, 31-32 transmission losses, 31,32 Episodic events, 88-90 Eragrostis lehmanniana (Lehmann's love grass), 141, 213, 214, 281, 294, 297, 314 Eremias lineoocellata, 215, 216 Eremias lugubris, 215, 216 Erioneuron pulchellum (fluff-grass), 235 Eucalyptus (mallee), 73, 75, 76, 267 Eulimnadia texana (clam shrimp), 223 Euphorbiaceae, 137 Evaporative heat loss, 144 Evaporative water loss, granivorous rodents, 149

Facilitation, 109-110 Fagonia, 22 Falco femoralis (Aplomodo falcon), 295 Fallugia paradoxa, 257 Fat stores, 142, 144 Feedbacks, 8 Feeding deterents, 185-186,189,192, 193,195 Felis concolor (mountain lion), 288 Ferocactus acanthodes, 107,138 Festuca, 259 Festuca pallescens, 172 Fiber production, 322 Field capacity, 15 Fire, 288 Fire ants {Solenopsis xyloni), 208 Flatlands, 33-36 Flood, 88-89 Floodplains, 34,37 Flourensia cemua (tarbush), 33, 73, 74, 76, 186,190,192,193,196,257 Fluff-grass {Erioneuron pulchellum), 235 Fluvial transport, 84-85 Food crops production, 319, 321-322 Food webs, 11, 185 below-ground, 220-222

ephemeral ponds/lakes, 92 Foquieria, 105 Foquieria splendens (ocotillo), 125, 257 Foraging pits, 100, 111, 253 Formica perpilosa, 113 Fox, 130 omnivory, 185 Fragmentation processes, 237, 238, 243 photodegradation, 240 Frankia, 260 Franseria (burr sage), 29, 76, 190 Franseria dumosa, 126 Frontal rainfall, 52 Frugivory, 206 Functional groups, 313 primary production, 172-177 Fungal decomposition, 249

Gazella dorcas (Dorcas gazelle), 190 General circulation models, 44 Geoffroea decorticans, 277 Geolycosa rafaelana, 200 Geomorphology, 17, 21 productivity effects, 168 see also Landform Germination delay/inhibition, 124 Gilgai relief, 36 Gnathamitermes tubiformans, 133, 249 Goannas (varanid lizards), 101 Gobi Desert, orographic effects, 53 Goods and services provision, 8, 319, 320 Gopherus agassizii (desert tortoise), 142 Granivory, 206-212 ant-rodent interactions, 213,214-215 impact on ecosystem structure/processes, 213-215 Grasses adaptations, 141 dune vegetation, 38 herbivory, 190 compensatory growth response, 202-203, 204 impact of kangaroo rat seed consumption, 213-214 photosynthetic pathways, 141 rainfall infiltration/run-off, 79, 80 rooting systems, 254 Grasshopper, 190-191, 193, 217, 268 Grasshopper mouse (Onychomys leucogaster), 209 Grassland banded vegetation patterns, 33

Index conversion to coppice dunes, 276 granivory, 208 invasion by shrubs (desertification changes), 276-277, 278, 279, 280-281, 282, 284, 286, 287 rehabihtation efforts, 295, 297 seed germination-estabUshment sites, 99 surface active arthropods, 199 Gravel plains, 36 Grayia, 140 Grazing, 202-206 associated desertification changes, 278, 279, 280-282, 283-284 optimization conceptual model, 202-203 soil effects, 202, 204-205 species preferences, 203, 204 vegetation effects, 202-204 Great Basin Desert desertification changes, 287-288 herbivory, 188 indicators for monitoring/assessment, 314 orographic effects, 53 productivity, 163 rodent-related seed traps/germination sites, 99 seasonal climate, 46 vegetation, 53, 162 Green rabbitbush (Chrysothamnus viscidiflorus), 188 Grey fox {Pseudalopex griseus), 206 Grey fox (Urocyon cinereoargenatus), 288 Gross rainfall, 72 Gular flutter, 151 Gutierrezia (snakeweed), 76, 186 Gypcrete, 36 H Hairy-nosed wombat {Lasiorhinus latifrons), 111 Halogeton glomemtus, 287 Haloxylon, 138 Hamadas, 36 Hammada, 22, 138, 167, 168 Hare, 130 Herbivory, 17, 185 canopy insect communities, 192-198 foliage quality determinants of abundance, 197 compensatory plant growth, 188,191 foliage chewers/browsers, 185-191 large grazers, 202-206 patch temporal changes, 118 physiological adaptations

333

mammals, 143, 144-145 reptiles, 141, 142 plant deterrent spinesence, 189 plant tissue water content relationship, 188, 189,190 plant toxins/feeding deterents, 185-186, 189, 192, 193, 195 protective nurse plant associations, 109 rainfall infiltration/run-off effects, 82 seed dispersal, 206 sucking arthropods, 193, 197 Hespemloe, 322 Heuweltjies, 111 Hilaria mutica (tabosa grass), 33, 69 Hilaria rigida, 107, 109 Hillslopes, 22-24 Hodotermes mossambicus, 324 Horned lizard (Phrynosoma), 129, 218, 220 Hydraulic lift, 138-139 Hydric stress, soil surface/subsurface, 58, 61 Hydrological landscape, 73 Hyper-arid regions, 1 ant communities, 201-202 decomposition of buried materials, 245 definition, 12 ephemeral stream vegetation, 32 productivity, 170-171 Hyperolius, 127 Hystrix austro-africanae (porcupine), 100 I Ibex, 100 Ifloga spicata, 99 lmp3ilsL (Aepycews melampus), 189 Indian desert drought, 44 seasonality, 49 Indian rice-grass (Oryzopsis hymenoides), 99 Indicators, 306,310-315 abundance/species richness, 313-314 animals, 314 criteria, 311 qualitative, 307, 308-309 sensitivity to disturbance/stress, 312 Indus, 37 Infiltration of water, 72,78-84 animal activities/macropore formation, 82-84, 110 arthropod activity effects, 113 termites, 251 catenas/watersheds, 85 ephemeral stream channels, 92 measurement, 309

334

Index

Inputs, 6, 7 Interception rainfall, 72, 73-76 Interception zones, 118-119 Intermediate disturbance hypothesis, 185 Intraguild predation, 185 Invertebrate adaptations, 142-143 Iranian Desert, aridity index (/^), 46 Ironwood (Olyneya tesota), 29, 109 Irrigation experiments, 175-176 Isomeris aborea, 126 J Jsickrahbit (Lepus), 100, 130, 138, 186, 188, 206 Jatropha dioica, 105 JaveUna (Pecari tajacu), 206, 321 Jet stream flow, 44 K Kalahari Desert bird-mediated fruit dispersal, 101 dune vegetation, 38 lizard communities, 215 seasonality, 49, 51 wind erosion, 67-68 Kangaroo, 100, 147, 323 Kangaroo rat (Dipodomys), 45, 145, 213, 217-218 reproduction, 149 Karoo animal-produced pits, 100 ant granivory, 211 monitoring/assessment systems, 306, 307, 309 seasonality, 49, 51 shrub herbivory, 189 shrub invasion, 287 Kelso sand dunes, 38 Keystone predation, 185 Keystone species, 294, 313 Kit fox, 150 Krameria, 140 Krameria parvifolia, 175 Kudu (Tragelaphus strepsiceros), 189 L Lactation, 147, 149 Lagostomus maimus, 206 Lake Eyre, 37 Landform, 1 7 , 2 1 ^ 0 Landform stability, 40 Landscape, 2, 3, 4, 5, 9-10

desertification consequences, 282-289 hydrological, 73 nutrient ditribution patterns, 252-257 water balance, 72-73 Landstat Thematic Mapper, 315 Larrea, 140, 190, 193 Larrea cuneifolia, 186,193 Larrea tridentata see Creosotebush Lasiorhinus latifrons (hairy-nosed wombat), 111 Leaf adaptations, 123, 136 Leaf growth, 140 Leaf nutrient resorption, 257-258 Leaf size, 123 Leaf succulents, 138 Legume nitrogen fixation, 260 Lehmann's love grass (Eragrostis lehmanniana), 141, 213, 214, 281, 294, 297,314 Lepidium lasiocarpum, 124 Le/?M5 Gackrabbit), 100, 130, 138, 186, 188, 206 Lepus califomicus (black-tail jackrabbit), 186, 188 Lichen crusts disruption by animal activities, 82 nitrogen fixation, 259 rainfall infiltration effects, 81 soil nutrient enrichment, 259-260 see also Crytogamic crusts Life form, 5 Life history, 5 adaptation to seasonality, 45 Lignin, 238, 239, 240, 245, 249 Linear (longitudinal) dunes, 38, 39 Litter accumulation around shrubs, 70-71, 103 dams/microterraces caused by run-off, 86-87 decomposition, 238-239 rates/mechanisms, 239-244 soil processes, 245-247 measurement for monitoring/assessment, 309 termite removal, 249 water content, 58 Lizards, 129 desert communities, 215 herbivores, 215 patch refugia, 54 physiological adaptations, 141 predators, 215-220,252 prey, 216-217

Index soil enrichment, 252 rainfall relationships, 217-218 secondary production, 181, 183 Lobo wolf (Canis lupus), 288 Local extinction, 4 Loess accumulation, 39 Long-term measurements, 90 Longitudinal (linear) dunes, 38, 39 Lophocereus schottii (senita), 27 Lotus, 260 Lupinus, 260 Lycium, 22, 140, 174 Lygus, 193 M Macropores, 82-84, 87, 110, 251 Macropus robustus, \A1 Macropus rufus (red kangaroo), 147 Magnesium, 117 Mallee (Eucalyptus), 73, 75, 76, 267 Mammalaria, 109 Mammals adaptations behavioural, 129-131 physiological, 143-150 burrows, 129-130 crepuscular activity cycle, 130 granivory, 207,208-210 omnivory, 185 patch refugia, 54 secondary production, 181,182,183 soil disturbance patches (pedturbation), 110, 111-112 Management strategies, 319 Markovian Dynamics, 105-106 Marsubiumvulgare, 281 Marsupial reproduction, 147 Meat production, 319, 322, 323, 324 Mephitis {sk\xr±), 288 Meriones crassus, 212 Mesas, 24 Mesquite see Pwsopis; Prosopis glandulosa; Prosopis juliflora Metabolic water production, 145 granivorous rodents, 149 Microchmate, 58-61 soil, 247 Migratory locust {Schistocerca gragaria), 196 Millipedes (myriopods), 131 Mineralization processes, 237-238, 243 soil microfauna activities, 247-249 Mites see Acari

335

Moina wierzejski, 223, 225 Moisture index (/), 46 Mojave Desert alien species expansion, 288 herbivory, 186 productivity, 164, 165, 174, 175 seasonal climate, 46 seasonality of animal reproduction, 48, 50 seed densities, 98 vegetation, 107, 109, 140, 162 Mole rats, 111 Molhsols, 15 Moloch horridus, 220 Monitoring/assessment systems, 305-315 data, 309 definitions, 306 existing systems, 306-309 indicators, 306, 307, 308-309, 310-315 Mormon tea. (Ephedra trifurca), 186 Morphological variation, single-species patches, 103-104 Mosaics, 3, 5, 9, 97, 104-106 grazing effects, 204 Mosquitoes (Aedes), 223 Mountain and basin deserts, 21, 22 Mountain lion (Felis concolor), 288 Mountains, 22-24 orographic effects/rainfall spatial variability, 53 Muhlenbergia porteri, 3 Mulga woodland, 73, 75, 101, 200 arthropod activities, 110 banded vegetation patterns, 33, 34 ephemeral lakes, 92 productivity, 160-161 Mulhenbergia porterii, 109 Mycorrhizae, 266-269 Myriopods (millipedes), 131 Myrmecocystus, 218 N Namib Desert, 40 ant granivory, 211 aridity index (/^), 47 decomposition of buried materials, 245 herbivory, 186 productivity, 171 rainfall, 56 seasonality, 49 surface active arthropods, 201

336

Index

Negev Desert decomposition processes, 243-244 episodic events frequency, 88 herbivory, 190, 196 loess accumulation, 39 productivity, 167, 168 rainfall, 52,56 surface active arthropods, 200-201 vegetation, 51 Nematodes, 221,264 adaptations, 143 anhydrobiosis, 247 decomposition activity, 248,251 Neotoma (woodrat), 186, 189 New Mexico desertification changes, 278 orographic effects/rainfall spatial variability, 53 rainfall predictability, 54 wind dispersal of seeds, 98 Nile, 37 Nitrogen availabihty, 14, 16-17, 235-236, 264 arthropod activity effects, 113, 117 mycorrhizal associations, 268 nitrogen-fixing legumes, 254-255 primary productivity relationship, 175, 176 soil organic matter relationship, 255, 257 Nitrogen cycle, 258-266 ammonia volatilization losses, 262-263 denitrification, 262, 263 measurement of processes, 309 nitrogen mineralization/immobilization rates, 264-266 Nitrogen fixation, 258-262 atmospheric inputs, 261-262 free-living bacteria, 258, 261 root nodule symbionts, 254, 258, 259, 260 Nitrogen mineralization, 264-266, 309 Nitrogen resorption, 257, 258 efficiency, 136 Nitrogen-fixing legumes, 254-255, 260 Aô/mfl! (palmilla), 322 Nomadism in birds, 131 North Africa, aridity index (/^), 46, 47 North American desert desertification, 278-279 desertified landscape rehabilitadon, 295-297 ephemeral lakes, 90, 92 lizard communities, 215 rodent granivory, 210, 211 Notiosorex crawfordii, 209 Nurse plants, 107,109-110

environmental modification, 105 Nutrient levels animal disturbance of soil, 110 arthropod activity effects, 113, 117 distribution patterns, 252-257 Nutrient limitation, 235-236 productivity modelling, 164 rain use efficiency, 162 Nutrient resorption, 257-258 drought deciduous perennials, 125-126 Nutrient retention, 310 Nutrient/materials cycling, 6, 8, 16 O Oberlander index (aridity index; /^), 46-47 Ocotillo {Foquieria splendens), 125, 257 Octodon degus (degu), 214 Odocoileus virginianus (white-tailed deer), 206, 321 Olyneya tesota (ironwood), 29, 109 Omnivory, 185 Onychomys leucogaster (grasshopper mouse), 209 Onychomys torridus, 209 Opuntia, 101-102, 105, 138, 206 commercial exploitation, 321 Opuntiaficus-indica, 321 Opuntia fulgida, 109 Opuntia leptocaulis (pencil cholla), 109 Opuntia polyacantha, 56 Opuntia rastera, 105 Opuntia streptacantha, 104, 105 Opuntia violacea, 188 Organ-pipe cactus {Stenocereus thruberi), 27 Organic material redistribution accumulation in animal-produced pits, 100 ant activities, 113 ephemeral lakes, 92 run-off, 86-87 termite activities, 117 wind erosion, 70 Organism, 2 Organism-environment interactions, 8 Organization levels, 2 Orographic effects, 53 Orycteropus afer (aardvark), 100 Oryx (Oryx gazella), 143, 144, 186, 323 Oryzopsis hymenoides (Indian rice-grass), 99 Osteospermum sinuatum, 189,195 Ostrich (Struthio camelus), 151, 324 Otocyon megalotis (bat-eared fox), 100 Outputs, 6, 7

Index

P (precipitation)/ET (mean annual evapotranspiration) aridity index, 11-12 Palmilla (iVo/ma), 322 Palo verde (Cercidium), 29, 80, 105, 107, 109, 173 Panicum, 22 Panicum obtusum, 222 Panicum turgidum, 37 Pans see Ephemeral ponds/lakes Panting in birds, 150 Paper making, 322 Patches, 3, 4, 5, 6, 7, 9, 17, 97-119 animal-produced, 110-117,184 competition, 106-107 grazing effects, 204 limiting resources location, 21 Markovian Dynamics model, 105-106 multi-species, 104-106 nutrient heterogeneity, 253 refugia, 54 scaling-up problems, 18 seed germination-establishment sites, 97-102 single species, 102-103 morphological variation, 103-104 temporal dynamics/feedbacks, 117-119 Pavements, 36-37 Pecari tajuca (collared peccary; javelina), 206, 321 Pencil cholla (Opuntia leptocaulis), 109 Percolation, 72 Perennial plants adaptations, 133-151 drought-deciduous, 125-126 functional groups, 137 root systems, 254 Pericopsis, 260 Pheidole, 208 Pheidole rugulosa, 213 Phenolic compounds, 192, 238 Phosphorus availability, 236, 266 arthropod activity effects, 113, 117 mycorrhizal associations, 268 resorption efficiency, 257 Photodegradation processes, 240 Photosynthesis, 139 pathways, 123, 134 Phreatic layer, 73 Phrynosoma (homed hzard), 129, 218, 220 Phrynosoma cornutum, 129, 218, 220 Phrynosoma modestum, 218 Phyllomedusa, 128

337

Physical conditions, 7 Physiological threshold hmits, 54-55 Piedmonts, 24-30 Piospheres, 204-205, 323 Plant adaptations see Adaptations Plant toxins/feeding deterents, 185-186, 189, 192, 193 Plant-weather interactions, 5 Plaruroctonus mesaenis, 201 Platform desert vegetation, 22 Playas see Ephemeral ponds/lakes Plectrachne schinzii, 39 Pogonomyrmex, 129, 218 Pogonomyrmex barbatus, 113 Pogonomyrmex desertorum, 208 Pogonomyrmex occidentalism 268 Pogonomyrmex rugosa, 113 Polypedilum vanderplanki, 143 Polyphenols, 189 Population, 2, 3 Porcupine (Hystrix austro-africanae), 100 Porcupine grass (spinifex), 39 Potassium reptile physiological adaptations, 141, 142 soil levels, 113 Potential evapotranspiration (PET):mean rainfall ratio, 51-52 Prairie dog (Cynomys ludovicianus), 288, 289 Predators, 17, 185 lizards, 215-220 poisoning campaigns for livestock introduction, 288 rodent prey, 212 surface active arthropods, 199-200, 201 Primary production, 157-177,310 below-ground, 162 comparisons with mesic ecosystems, 160-161 desertification effects, 276 extreme deserts, 170-171 grazing response, 202-203 landscape relationships, 166-170 net, 157 measurement, 157-159 nutrient limitation, 162, 235-236, 237 rainfall relationship, 51-52, 163-164, 165, 166, 167, 168, 170, 171-172 experimental approaches, 175-176 species/functional groups, 172-177 see also Annual above ground net primary production Prong-horned antelope, 186 Prosopis, 103, 105, 173, 254, 260

338

Index

Prosopis flexuosa, 166, 206 Prosopis glandulosa (mesquite), 32, 33, 39, 51, 73, 76, 99, 107, 113, 118, 140, 177, 189, 191, 196, 198, 206, 207, 255, 257, 258, 276, 280, 284, 290, 294, 315 Prosopis juliflora, 29 Prostigmatid mites, 221 decomposition activity, 248 Protozoa, soil microfauna, 221, 264 decomposition processes, 247,251 Pruning activities, 186,188 Psammomys obesus, 149 Pseudalopex griseus (gray fox), 206 Psocoptera, 221 Psorothemnus spinosus, 260 Pterocarpus, 260 Ptewniapallens, 195 Pulse-reserve paradigm, 12-14, 16, 17, 157, 170,223

Qualitative indicators, 307, 308-309 R Rabbit, 130, 186, 188 Radiative energy flux, 144 Rain use efficiency, 161-162 Rain-splash, 77-78, 103 Rainfall, 14, 17 aridity index (/^), 46-47 autoecological hypothesis, 12-13, 14 characteristics, 17 convectional, 52 decomposition relationship, 240, 243, 244, 245,247 soil microfauna activities, 247 duration, 55-58 episodic storms, 88, 89 frequency/return time, 17, 55-58 frontal, 52 gross, 72 infiltration, 72,78-84 intensity, 17,55-58 intercepfion, 72, 73-76 nitrogen availability relationship, 14, 264 nutrient deposition, 262 partitioning, 72 percolation, 72 predictabiUty (variabiUty), 53-55 productivity relationship, 163-164, 165, 166,167, 168, 171-172 experimental approaches, 175-176 extreme deserts, 170-171

pulse-reserve model of ecosystem function, 13, 14, 16 quantity, 17 redistribution, 4,71-77 exchanges among landscape units, 84-86 exchanges within landscape units, 86-88 following loss of grass cover, 280, 284, 286 run-off, 72,78-84 seasonality, 17,45,48-50,51 spatial variability, 52-53 stemflow, 72, 73-76 throughfall, 72,76-77 see also Wet season Ranching lifestyle, 292-293 Rangelands, 1,22,202 ecosystem dynamics, 282-283 monitoring/assessment systems, 306-307 ranching lifestyle, 292-293 Rantherium, 22 Rantherium sanveolens, 140 Red kangaroo {Macropus rufus), \A1 Regs, 36 Remote sensing, 306, 314-315 Reptiles adaptations, 129 physiological, 141-142 fat stores, 142 salt glands, 141 thermoregulatory behaviour, 129 Resilience, 9, 10-11 desertified landscape, 289-290 Resistance, 9 desertified landscape, 289-290 Retama retam, 166 Rhizobium, 254, 258, 260, 300 Rhizosheaths, 260-261 Rhus microphylla, 257 Rills, 35 Rio Grande, 37 Rivers, 37 Roadrunner, 151 Rock-strewn surfaces, 36 rainfall infiltration/run-off, 84 Rodents abundance patterns, 210, 212 following livestock introduction, 288-289 desertification vegetation changes, 281 granivory, 207, 208-210, 211-212, 213 impact on ecosystem structure/processes, 213-215 metabolic rates, 149-150

Index physiological adaptations, 148-150 seasonality of reproduction, 48, 50 soil disturbance. 111 seed germination sites, 98-99 Rooting patterns/root systems, 123, 137, 138, 140 measurement for monitoring/assessment, 309 nutrient status effects, 253-254 Roots biomass estimates, 162 decomposition, 239,251,264 soil processes, 245-247 depth, 138,140,177 Rope making, 322 Rotifers, 224 Run-off, 72,78-84, 161,310 catenas/watersheds, 85 ephemeral lake filUng, 90, 92 exchanges among landscape units, 84-86 exchanges within landscape units, 86-88 measurement, 309 microtopographic effects, 85, 86 organic matter redistribution, 86-87 productivity relationship, 164, 166, 167 Runnels, 35, 37

SsLgQbrush (Artemisia tridentata), 107, 188 Saguaro (Carnegia gigantea), 27, 29, 105, 107,109,288 Sahara ephemeral stream transmission losses, 32 lizard communities, 215 nutrient availabihty, 236 seasonality, 49, 51 Sahel drought, 44 nutrient availability, 235, 236 seasonality, 49, 51 social/economic impact of desertification, 276, 277, 291-292 termite activity, 82-83 wild food plants, 322 S aline environments, 138 Salsola, 287 Salsola vermiculata, 140 Salt glands, 141, 151 Sand dunes, 37-40 detritivores, 198 microphytic crusts, 81 productivity, 177 rainfall infiltration, 78,81

339

stabilization with artificial vegetation, 299 surface active arthropods, 198 wind-related effects, 67, 68 Sand features, 37-40 Sand partridge, 151 Sand plains, 22,34-35 Sand seas, 22,37 Sarcobatus, 29 Sauromalus obesus (chuckwalla), 141, 142 Scaling, 18 Scaphiopus {^^didtfooiiodid), 126-127, 128, 129, 226 Scaphiopus bombifrons, 224, 225 Scaphiopus couchi, 223, 224, 225 Scaphiopus (hammondi) multiplicatus, 127, 128,223,224,225 Scarabeus christatus, 198 Schistocerca gragaria (migratory locust), 196 Scleropogon brevifolia, 3, 69 Scolopendra, 132 Scorpio marus palmatus, 201 Scorpions, 198,200-201 Scouring events, 89, 90 Sea surface temperature, 43, 44 Seasonahty, 45-46, 48-52 animal reproduction, 48, 50 predictability, 53 productivity modelling, 163 vegetation effects, 50-51 Sebkas see Ephemeral ponds/lakes Secondary production, 181-185,310 Seed consumption see Granivory Seed dispersal animals, 101-102 birds, 99, 206 carnivores, 206 herbivores, 206 desertification vegetation changes, 281, 284 wind, 97-98 Seed dormancy polymorphism, 124-125 Seeds emphemeral plants, 124 germination delay/inhibition, 124 germination-establishment sites, 97-102, 105,214 nurse plant protective effects, 109 Seidlitzia, 138 Seira bipunctata, 200 Semiarid regions, 1 definition, 11, 12 Senita (Lophocereus schottii), 27 Sheep, 143, 144

340

Index

Shield deserts, 21 vegetation, 22-23 Shrubs alluvial fans/bajadas, 29 chewing-browser herbivory, 186, 188-190, 191 deep-rooted, 138-139, 140, 177 drought deciduous, 125-126 ephemeral stream channel, 32 flatlands, 34,35 grassland invasion (desertification changes), 276-277, 278, 279, 280-281, 282, 284, 286, 287 rehabilitation efforts, 295, 297 resistance/resilience, 289-290 hydraulic lift, 138-139 leaf growth phenology, 140 litter accumulation, 70-71,103 morphological impact on stemflow, 103 patch development, 99 physiological adaptations, 138 productivity, 172, 173 rainfall infiltration/run-off, 80-81, 86 root systems, 254 seed dispersal by herbivores, 206 shield/platform desert, 22 soil nutrient accumulation ('fertile islands'), 102, 252-253 desertification processes, 284, 286 spatial distribution, 107 Silcrete, 36 Silver fish, 217 Simmondsia chinensis, 191 Skunk (Mephitis), 288 Smithurides pumilis, 200 Snakes, 129 SnakQWQed (Gutierrezia), 76,186 Social impact of desertification, 276, 277, 291-293 Soil, 4, 14-16 alluvial fans/bajada vegetation, 27-30 animal disturbance patches, 110, 214, 253 arthropod activity-related enrichment, 113, 114-117 bacteria denitrification, 262-263 nitrogen fixation, 259, 261 desert types, 15-16 erosion, 5 cohesive resistance to wind, 68-69 raindrop splash, 77-78 food webs, 220-222 gradients along catenas/watersheds, 85

grazing effects, 202, 204-205 macrofauna, 220 mesofauna, 221 anhydrobiosis, 143 microclimate, 247 microfauna, 221 decomposition/mineralization activities, 247-249, 264 soil quality indicator, 314 mites see Acari moisture, 5, 15, 16 water balance parameters, 46 wind erosion susceptibility relationship, 66 nutrient accumulation beneath shrubs ('fertile islands'), 102, 252, 253 organic matter, 16, 309 nutrient availability relationship, 255, 257 termite abundance relationship, 250-251 rainfall partitioning, 72 infiltration, 78-84 macropores, 82-83 run-off, 86 stemflow water, 74-76 surface/subsurface temperature, 58, 61 texture, 16 vadose water, 73 water potential, 15,16,73 Solar radiant energy, 58 Solenopsis xyloni (fire ants), 208 Solfugids, 200 Sonoran Desert alien species expansion, 288 herbivory, 190, 191 nitrogen fixing legumes, 260 plant community dynamics, 105-106 productivity, 163, 177 seasonality, 46, 50, 51 seed densities, 98 vegetation, 27, 29, 107, 109, 140 South African desert aridity index {I^), 47, 48 desertified landscape rehabilitation, 298-299 South American desert aridity index (/j^), 47,48 desertification processes, 287 desertified landscape rehabilitation, 300 lizard communities, 215 rodent granivory, 210 seasonality, 50, 51 South Asian desert, aridity index (7^^), 47 Southern Oscillation, 44

Index SpsidQfoot tosid (Scaphiopus), 126-127, 128, 129, 226 Spatial scale, 2, 4 Spatial variation, 11 climate, 52-53 decomposition processes, 244 desertification processes, 286-287 plant distribution, 106-107 Spiders, 199,200,217 Spinesence, 189 Spinifex grassland, 39 Splash erosion-kinetic energy of raindrops, 77-78 Sporobolus, 141,297 Sporobolus flexuosus, 6 Stability, 9,10,11 Stem adaptations, 136 Stem photosynthesis, 123 Stem succulents, 138 Stemflow, 5,72,73-76 Stenocereus thruberi (organ-pipe cactus), 27 Stipagrostis amabilis, 38 Stipagrostis obtusa, 38 Stomatal adaptations, 123 Stony surfaces, 36-37 Storms, 88,89,310 debris flows, 89 fluvial flows, 89 primary production modelling, 163, 164, 166 watershed scouring events, 89, 90 Streams, 37 see also Ephemeral streams Streptocephalus texanus, 223 Struthio camelus (ostrich), 151, 324 Suaeda, 29, 138 Subhumid zone, 12 Succulents, 123, 137-138 Australian semi-arid regions, 44-45 leaf, 138 saline environments, 138 stem, 138 Sucking herbivorous arthropods, 193, 194 Surface active arthropods, 198-202 Surface crusts measurement for monitoring/assessment, 309 rainfall infiltration effects, 81-82 resistance to wind erosion, 68 see also Algal crusts; Cryptogamic crusts; Lichen crusts Surface layer, hydrological landscape, 73 Sylvilagus auduboni (desert cottontail), 188

341

Symbiosis, 10 Symbiotic nitrogen fixation, 254, 258, 260

Tabosa grass {Hilaria mutica), 33, 69 Tadpole shrimp {Triops longicaudatus), 223 Tail shading behaviour, 130 Tannins, 186, 189, 192, 195 Tarbush {Flourensia cernua), 33, 73, 74, 76, 186, 190, 192, 193, 196,257 Taxidea taxus (badger), 206 Tegeticula yuccasella (yucca moth), 204 Temperature organic materials decomposition, 240 seasonal timing, 45 soil surface/subsurface, 58, 61 Temporal heterogeneity, 11 desertification processes, 286-287 Tenebrionid beetles, 142-143, 198, 199 Termites, 84, 87, 220-221, 294 behavioural adaptations, 133 decomposition activities, 249-251 exploitation for food production, 324 gut symbionts, 249 lizard predation, 216-217,252 plant productivity impact, 113 soil effects, 113,114-117 burrowing disturbance patches, 110, 111 macropore production, 82-83,110, 251 nutrient transport, 110 soil organic matter relationship, 250-251 vegetation effects, 117 Themeda triandra. 111 Themnocephalus platyurus, 223 Thermal energy balance, 123, 143-144 birds, 150 Thermoregulatory behaviour, reptiles, 129 Throughfall, 72, 76-77 Time scales, 2 Tolerance thresholds, 3, 4 Topography productivity effects, 168, 170 rainfall spatial variability, 53 Torrey ephedra (Ephedra torreyand), 186 Traganum, 138 Tragelaphus strepsiceros (kudu), 189 Transmission losses ephemeral lakes, 92 ephemeral streams, 31, 32 Transverse dunes, 38

342

Index

Trees alluvial fans/bajadas, 29 Australian semi-arid regions, 45 banded patterns, 33 hydraulic lift, 138-139 physiological adaptations, 138 productivity, 173 rainfall infiltration/run-off, 80-81, 86 root systems, 254 seed dispersal by herbivores, 206 shield/platform desert, 22 soil nutrient accumulation ('fertile islands'), 252 Trimerotropis pallidipennis, 190, 191 Trinervitermes trinervoides. 111 Triodia basedowii, 39, 217 Triodia pungens, 39 Triops longicaudatus (tadpole shrimp), 223 Trophic relationships, 11, 16-17, 181 Tunisian desert desertified landscape rehabilitation, 299-300 productivity, 169 rainfall, 52,53 vegetation, 140 U Urocyon cinereoargenatus (grey fox), 288 Uromastix, 142, 215 Uromastix acanthinurus, 141 Uta stansburiana, 181,216

Vadose layer/vadose water, 73 Varanus gouldii, 200 Variable stem death, 140 Vegetation alluvial fans/bajadas, 27-29 arcs, 33 banded patterns, 33 catena/watershed sediments, 85 ephemeral streams, 31-32 grazing effects, 202-204 platform deserts, 22 rainfall distribution effects, 73 infiltration/run-off, 78, 79-81, 85, 86 kinetic energy reduction, 78 rainfall seasonality effects, 50-51 rainfall spatial variability effects, 53 rodent/ant seed consumption impact, 213-215 sand dunes, 38, 39

seasonality adaptations, 45 shield deserts, 21,22-23 termite mounds, 117 wind erosion reduction, 67-68, 70 Ventilago viminalis, 34 Vertebrate predators, 215-220 Vesicular-arbuscular endomycorrhiza, 266, 267-268, 300 Vulpes chama (Cape fox), 100 W Wadis, 22 see also Ephemeral streams Water balance birds, 151 carnivorous mammals, 150 landscape, 72-73 parameters, 46-48 large herbivorous mammals, 145-147 rodents, 148-149 Water infiltrating landscapes, 73 Water loss reduction, 123 amphibians, 126-127 Water retention, 310 Water spreading landscapes, 73 Water use efficiency, 136, 160 see also Rain use efficiency Watersheds, 4, 9, 17 fluvial transport, 84-85 scouring events, 89, 90 Wet season, 48 potential evapotranspiration (PET):mean rainfall ratio, 51-52 productivity, 51-52 White-necked raven (Corvus cryptoleucus), 150 White-tailed deer {Odocoileus virginianus), 206, 321 Wilting point, 15 Wind dispersal of seeds, 97-98 Wind erosion, 65-71 animal activities impact, 68-69, 70 erosion cells model, 70 fetch length/threshold velocity relationship, 69-70 organic material redistribution, 70 soil cohesive resistance, 68-69 threshold wind speed, 66 vegetation-mediated reduction, 67-68, 70 Windstorms, 88 Winterfat (Ceratoides lanata), 53, 162, 188 Woodmt (Neotoma), 186,189

Index X Xerophytic food crops, 319 Xerus inauris (Cape ground squirrel), 130

Yucca, 87, 105, 133, 195, 322 Yucca elata, 102,204

Yucca moth (Tegeticula yuccasella), 204

Zinnia acerosa, 238 Zygogramma tortusa, 190, 196 Zygophyllum dumosum, 167, 168, 169

343

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